[M110][Fuchsia] Use kCompositing for render main on Fuchsia

Use kCompositing for the render main thread on Fuchsia to address
product performance issues.

This change only applies to M110. In M112+ we plan to use kMainThreadCompositingPriority feature, tracking that in http://go/crb/1431671.

Bug: 1431536
Bug: b/275114261
Change-Id: Ib9383c63d2b05e99f8a5c3de808e526e0fd8ed00
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4409000
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Auto-Submit: David Rees <drees@google.com>
Commit-Queue: David Rees <drees@google.com>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/branch-heads/5481@{#1393}
Cr-Branched-From: 130f3e4-refs/heads/main@{#1084008}

content/renderer/renderer_main.cc

@@ -264,6 +264,9 @@ int RendererMain(MainFunctionParams parameters) {
             features::kHandleRendererThreadTypeChangesInBrowser)) {
       RendererThreadTypeHandler::Create();
     }
+#elif BUILDFLAG(IS_FUCHSIA)
+    // TODO(crbug.com/1431671) Move to using kMainThreadCompositingPriority feature in M112+
+    base::PlatformThread::SetCurrentThreadType(base::ThreadType::kCompositing);
 #endif
 
     std::unique_ptr<RenderProcess> render_process = RenderProcessImpl::Create();