Use officially supported compile time thread local strike cache flag. (#191014)

With https://skia-review.git.corp.google.com/c/skia/+/1321976 landing,
we have a more officially supported way to opt into thread local strike
caches.

engine/src/flutter/skia/BUILD.gn

diff --git a/engine/src/flutter/skia/BUILD.gn b/engine/src/flutter/skia/BUILD.gn
index 99b69b8..b20346b 100644
--- a/engine/src/flutter/skia/BUILD.gn
+++ b/engine/src/flutter/skia/BUILD.gn
@@ -69,6 +69,10 @@
     defines += [ "SK_USE_PERFETTO" ]
   }
 
+  if (skia_enable_threadlocal_strikecache) {
+    defines += [ "SK_ENABLE_THREADLOCAL_STRIKECACHE" ]
+  }
+
   # Some older versions of the Clang toolchain change the visibility of
   # symbols decorated with API_AVAILABLE macro to be visible. Users of such
   # toolchains suppress the use of this macro till toolchain updates are made.

engine/src/flutter/skwasm/surface.cc

diff --git a/engine/src/flutter/skwasm/surface.cc b/engine/src/flutter/skwasm/surface.cc
index 65f6d9d..11fcf1d 100644
--- a/engine/src/flutter/skwasm/surface.cc
+++ b/engine/src/flutter/skwasm/surface.cc
@@ -41,16 +41,6 @@
 //    the Dart code, which will complete the future that was returned by the
 //    original Dart method call.
 
-// See https://github.com/flutter/flutter/pull/190048
-extern bool
-    gSkUseThreadLocalStrikeCaches_IAcknowledgeThisIsIncrediblyExperimental;
-
-namespace {
-__attribute__((constructor)) void UseThreadLocalStrikeCaches() {
-  gSkUseThreadLocalStrikeCaches_IAcknowledgeThisIsIncrediblyExperimental = true;
-}
-}  // namespace
-
 unsigned long Skwasm::GetRasterThread() {
   static unsigned long thread = []() {
     if (skwasm_isSingleThreaded()) {

engine/src/flutter/third_party/canvaskit/BUILD.gn

diff --git a/engine/src/flutter/third_party/canvaskit/BUILD.gn b/engine/src/flutter/third_party/canvaskit/BUILD.gn
index 92536dd..84952e6 100644
--- a/engine/src/flutter/third_party/canvaskit/BUILD.gn
+++ b/engine/src/flutter/third_party/canvaskit/BUILD.gn
@@ -138,6 +138,9 @@
       skia_use_no_png_encode = true
       skia_use_libpng_encode = false
 
+      # Always use thread local strike caches, since we use workers for multithreading
+      skia_enable_threadlocal_strikecache = true
+
       # skwasm is multithreaded
       wasm_use_workers = true
       wasm_prioritize_size = true