[web] Avoid absolute positioning for base CanvasKit canvas (#188337)

fix for #176327
Partial fix for #181511

**Problem**

With CanvasKit and semantics enabled, scrolling regular Flutter Web
content on iPhone WebKit browsers can be very janky. The base CanvasKit
canvas was always styled with `position:absolute`, which is expensive
for WebKit to scroll together with the semantics DOM.

**Fix**

Only platform-view overlay canvases keep `position:absolute`. The normal
base CanvasKit canvas now uses regular positioning, which fixes the
plain Flutter scroll case while preserving `HtmlElementView` stacking.

One limitation: mixed Flutter and `HtmlElementView` scenes may still
show iOS WebKit scroll jank because platform-view overlays still require
`position:absolute`.

**Demo**

| Before | After |
|---|---|
| https://flutter-demo-09-before.web.app |
https://flutter-demo-09-after.web.app |

Steps: open the page on iPhone Chrome or Safari, keep semantics enabled,
then scroll the page continuously.

Before: the on-screen jank log shows severe scroll jank, including runs
over 100 janky frames per 5 seconds.

After: the same regular Flutter content stays smooth, with low jank
summaries around 1 to 10 janky frames per 5 seconds in validation runs.