Expose WebGL2RenderingContext in worker

Currently WebGL2RenderingContext is not yet exposed in worker, which means
calling OffscreenCanvas.getContext('webgl2') fails in a worker.

This CL expose it with ExperimentalCanvasFeatures flag.
The test for this is upstreamed here:
https://github.com/KhronosGroup/WebGL/pull/2159

BUG=668727
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2531853003
Cr-Commit-Position: refs/heads/master@{#434966}

third_party/WebKit/Source/modules/webgl/WebGL2RenderingContext.idl

@@ -7,6 +7,7 @@
 [
     DoNotCheckConstants,
     SetWrapperReferenceFrom=canvas,
+    Exposed(Worker ExperimentalCanvasFeatures, Window StableBlinkFeatures),
     Custom=VisitDOMWrapper,
 ] interface WebGL2RenderingContext { };
 WebGL2RenderingContext implements WebGLRenderingContextBase;