webview: Use EnforceProcessCpuAffinity for little-only experiment

Switches the implementation of the --webview-force-little-cores switch
from base::SetProcessCpuAffinityMode to
content::EnforceProcessCpuAffinity. The latter periodically verifies
that the affinity setting remains active, to detect when the system
resets the affinity setting, and reapplies it if necessary.

Test: Run system WV shell with the flag & check chrome://histograms
Bug: 1111789
Change-Id: Icf4c360a36b8588086a0beb1caedd0e5869e3b59
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2502430Reviewed-by: Richard Coles <torne@chromium.org>
Reviewed-by: Laís Minchillo <laisminchillo@chromium.org>
Commit-Queue: Eric Seckler <eseckler@chromium.org>
Commit-Queue: Laís Minchillo <laisminchillo@chromium.org>
Auto-Submit: Eric Seckler <eseckler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821681}

android_webview/lib/aw_main_delegate.cc

@@ -51,6 +51,7 @@
 #include "content/public/common/content_descriptor_keys.h"
 #include "content/public/common/content_features.h"
 #include "content/public/common/content_switches.h"
+#include "content/public/common/cpu_affinity.h"
 #include "gin/public/isolate_holder.h"
 #include "gin/v8_initializer.h"
 #include "gpu/command_buffer/service/gpu_switches.h"
@@ -299,8 +300,8 @@ void AwMainDelegate::PreSandboxStartup() {
   if (process_type == switches::kRendererProcess) {
     InitResourceBundleRendererSide();
     if (command_line.HasSwitch(switches::kWebViewForceLittleCores)) {
-      base::SetProcessCpuAffinityMode(base::GetCurrentProcessHandle(),
-                                      base::CpuAffinityMode::kLittleCoresOnly);
+      content::EnforceProcessCpuAffinity(
+          base::CpuAffinityMode::kLittleCoresOnly);
     }
   }