CacheStorage: Use a sequence by default except on chromeos.

Field trials shows that using a sequence instead of the IO thread
progresses match times by 10% at P95 on windows.  Mac shows similar
progressions.  Android has mixed results and is somewhat neutral in
total.  ChromeOS, however, shows regressions across the board including
a 15% regression at P95 on match times.

Therefore, this CL enables the sequence feature on all main platforms
except ChromeOS.  Further experiments and trials will be run to launch
on the remaining platform in the future.

This CL also leaves the feature disabled on fuchsia for now.  Some
failing tests need to be investigated on that platform.

Bug: 960012
Change-Id: Ib4d379dff208a5363f73277ed4415d3e94dc17b1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2124606
Commit-Queue: Ben Kelly <wanderview@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#754550}

content/browser/cache_storage/cache_storage_context_impl.cc

@@ -11,6 +11,7 @@
 #include "base/task/post_task.h"
 #include "base/task/thread_pool.h"
 #include "base/threading/thread_task_runner_handle.h"
+#include "build/build_config.h"
 #include "content/browser/blob_storage/chrome_blob_storage_context.h"
 #include "content/browser/cache_storage/cache_storage_dispatcher_host.h"
 #include "content/browser/cache_storage/cache_storage_quota_client.h"
@@ -28,8 +29,18 @@ namespace content {
 
 namespace {
 
+// TODO(crbug/960012): Disabled on chromeos for now due to performance
+// regressions that need to be investigated.
+// TODO(crbug/960012): Disabled on fuchsia for test failures.
 const base::Feature kCacheStorageSequenceFeature{
-    "CacheStorageSequence", base::FEATURE_DISABLED_BY_DEFAULT};
+  "CacheStorageSequence",
+#if defined(OS_CHROMEOS) || defined(OS_FUCHSIA)
+      base::FEATURE_DISABLED_BY_DEFAULT
+};
+#else
+      base::FEATURE_ENABLED_BY_DEFAULT
+};
+#endif
 
 scoped_refptr<base::SequencedTaskRunner> CreateSchedulerTaskRunner() {
   if (!base::FeatureList::IsEnabled(kCacheStorageSequenceFeature))