CacheStorage: Enable sequence on ChromeOS.

This CL enables a feature to move cache_storage operations off
the IO thread and onto a sequence instead.  This feature has
previously launched on all other platforms.  It has been held
back until now on ChromeOS due to some measured performance
regressions.

After a couple months of experimentation we have not been able to
shift the regressions.  It appears there is just measurable delay
from the additional task that is run when mojo messages are targeted
off the IO thread.  There is not a lot we can do about this
unfortunately.

The regression show up in cache match time and body reading at P95.
Top level UMA such as FCP, however, are neutral.

Architecturally we are motivated to launch this in spite of the
regressions.  We do not want to support multiple threading models
into the future.  In addition, cache_storage can interfere with
other parts of the browser that also need to use the IO thread via
mojo message passing.

Therefore this CL proceeds with enabling the feature.

Bug: 960012
Change-Id: Ie9d5931ce9530dafe101d209a13f65ff22dffddb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2441125Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Ben Kelly <wanderview@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813395}

content/browser/cache_storage/cache_storage_context_impl.cc

@@ -28,17 +28,8 @@ namespace content {
 
 namespace {
 
-// TODO(crbug/960012): Disabled on chromeos for now due to performance
-// regressions that need to be investigated.
 const base::Feature kCacheStorageSequenceFeature{
-  "CacheStorageSequence",
-#if defined(OS_CHROMEOS)
-      base::FEATURE_DISABLED_BY_DEFAULT
-};
-#else
-      base::FEATURE_ENABLED_BY_DEFAULT
-};
-#endif
+    "CacheStorageSequence", base::FEATURE_ENABLED_BY_DEFAULT};
 
 scoped_refptr<base::SequencedTaskRunner> CreateSchedulerTaskRunner() {
   if (!base::FeatureList::IsEnabled(kCacheStorageSequenceFeature))