Apply kCompositing thread type to Media thread on Fuchsia.

Improve media performance on heavily loaded Fuchsia devices by applying
kCompositing thread type to the Media thread. The exact parameters of
this thread type are customized per-product using Fuchsia's scheduler
role mechanism.

In the long term, we would like to align on new thread types and
priorities for the media thread and related pool threads, suggested in
https://crrev.com/c/chromium/src/+/3920078/comment/545232d3_7203c331/

However, we have a release blocking performance bug that is confirmed to
be addressed by this minimal, Fuchsia-only change. Landing this in the
interim unblocks the release and buys time to align on the longer term
solution.

(cherry picked from commit 4dc925d)

Bug: 1174339
Bug: b/242564647
Change-Id: I1ad68f83c84e645386dd154c3d2f7f3e03ba1c7b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4103643
Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Corey Tabaka <eieio@chromium.org>
Cr-Original-Commit-Position: refs/heads/main@{#1082773}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4104857
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Srinivas Sista <srinivassista@chromium.org>
Owners-Override: Srinivas Sista <srinivassista@chromium.org>
Cr-Commit-Position: refs/branch-heads/5359@{#1202}
Cr-Branched-From: 27d3765-refs/heads/main@{#1058933}

content/renderer/render_thread_impl.cc

@@ -1590,6 +1590,9 @@ RenderThreadImpl::GetMediaThreadTaskRunner() {
 #if BUILDFLAG(IS_FUCHSIA)
     // Start IO thread on Fuchsia to make that thread usable for FIDL.
     base::Thread::Options options(base::MessagePumpType::IO, 0);
+    // TODO(crbug.com/1400772): Use kCompositing to address media latency on
+    // Fuchsia until alignment on new media thread types is achieved.
+    options.thread_type = base::ThreadType::kCompositing;
 #else
     base::Thread::Options options;
 #endif