[web] Repair RenderCanvas CSS size drift (#188797)

## What changed

This PR makes the stock `RenderCanvas` path repair stale inline canvas
CSS size even when the backing-store size and DPR have not changed.

Changes:

- Factor the expected logical canvas CSS size into a helper.
- Check actual `style.width` / `style.height` before the same-size early
return.
- Re-apply backing-store / DPR CSS size when the inline style is stale.
- Add regression coverage for `1206 x 2142 @ DPR 3`, repairing
`0.333333px` back to `402 x 714px`.

## Why

flutter/flutter#188723 tracks a Safari/WebKit Skwasm case where the
canvas backing store can be valid while the visible CSS/display rect
remains collapsed around `0.328125 x 0.328125px`.

The existing stock `RenderCanvas` code repairs CSS size on physical-size
changes and DPR changes, but not when only the inline CSS value is
stale. This PR makes that same-size path self-heal.

## Scope

This does not enable Skwasm by default, change WebKit policy, depend on
the multi-surface rasterizer, or by itself close all Safari/WebKit
default-policy gates.

## Validation

- `dart format`: passed.
- `git diff --check`: passed.
- Targeted `felt` browser test in the #188797 checkout: passed locally.
- Command: `DART_SUPPRESS_ANALYTICS=true HOME=/private/tmp
DART_SDK_DIR=/private/tmp/flutter-mobile-safari-skwasm-canvas-css-size-fix-upstream-worktree/engine/src/flutter/third_party/dart/tools/sdks/dart-sdk
./dev/felt test --gcs-prod --browser chrome --compiler dart2js
--renderer canvaskit --fail-early
test/engine/compositing/render_canvas_test.dart`
- Result: `chrome-dart2js-canvaskit-engine`, `3/3`, `All tests passed!`.
- Fresh `engine/src/out/wasm_release/flutter_web_sdk` build from the
#188797 worktree: passed locally.
- `dev/benchmarks/macrobenchmarks` built as web wasm/profile using the
local `wasm_release` SDK: passed locally.
- Guardrail: preserved SDK/app build logs and generated bundle were
checked for `FLUTTER_WEB_SKWASM_FORCE_MULTI_SURFACE_RASTERIZER`; the
string was absent.
- Simulator MobileSafari, Skwasm, `variant=none`, no forced CSS sizing,
`bench_card_infinite_scroll`: `probe-complete`, `errors: []`, loaded
`skwasm_heavy`, `main.dart.mjs`, and `main.dart.wasm`,
`crossOriginIsolated=true`, canvas backing `1206 x 2142`,
CSS/display/style `402 x 714` / `402px x 714px`.
- Physical iPhone MobileSafari, Skwasm, `variant=none`, no forced CSS
sizing, `bench_card_infinite_scroll`: `all-benchmarks-complete`,
`errors: []`, loaded `skwasm_heavy`, `main.dart.mjs`, and
`main.dart.wasm`, canvas backing `1206 x 2142`, CSS/display/style `402 x
714` / `402px x 714px`.
- Physical iPhone MobileSafari, Skwasm, `variant=none`, no forced CSS
sizing, `bench_simple_lazy_text_scroll`: `all-benchmarks-complete`,
`errors: []`, loaded `skwasm_heavy`, `main.dart.mjs`, and
`main.dart.wasm`, canvas backing `1206 x 2142`, CSS/display/style `402 x
714` / `402px x 714px`.

Physical iPhone page telemetry reported `crossOriginIsolated=false` even
though the harness served isolation headers. Skwasm still loaded through
the WebKit allowlist, so this is local physical
geometry/benchmark-completion evidence, not a cross-origin-isolation
pass.

Related issue: flutter/flutter#188723.
Broader Safari/WebKit Wasm/Skwasm tracking: flutter/flutter#178893.

---------

Co-authored-by: Harry Terkelsen <1961493+harryterkelsen@users.noreply.github.com>