Revert 141514 - Remove CachingValueStore in favor of an async ValueStoreFrontend.

Revert 141514 - Remove CachingValueStore in favor of an async ValueStoreFrontend.

Caused massive number of use-after-free crashes on ClusterFuzz.

After actually seeing the use cases for the value store in extension code, I
realized that an async version would be more appropriate. This one will use
less memory as well, since we won't need to keep the cache in memory.

BUG=123366
TEST=no


Review URL: https://chromiumcodereview.appspot.com/10539073

TBR=mpcomplete@chromium.org
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=141603