[HeapMojo] Make HeapMojoWrapperMode::kWithContextObserver default

Recent experiment shows that there is no huge impact on crash rates. So this CL is going to change the default mode to use HeapMojoWrapperMode::kWithContextObserver.

Bug: 1049056
Change-Id: I801a57fdacc6195b95aef9d9fa6f1a896595f9b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2383406Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Minoru Chikamune <chikamune@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803272}

third_party/blink/renderer/platform/mojo/features.cc

@@ -7,5 +7,5 @@
 namespace blink {
 // HeapMojo experiments to deprecate kWithoutContextObserver
 const base::Feature kHeapMojoUseContextObserver{
-    "HeapMojoUseContextObserver", base::FEATURE_DISABLED_BY_DEFAULT};
+    "HeapMojoUseContextObserver", base::FEATURE_ENABLED_BY_DEFAULT};
 }  // namespace blink