Unrevert 141514: Remove CachingValueStore in favor of an async ValueStoreFrontend.

Unrevert 141514: Remove CachingValueStore in favor of an async ValueStoreFrontend.

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, 132203
TEST=no

Changes: Fixed a use-after-free bug.

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=141745

[Matt Perry, 2012-06-12 20:42:16]

http://codereview.chromium.org/10533116/diff/1/chrome/browser/value_store/val...
File chrome/browser/value_store/value_store_frontend.cc (right):

http://codereview.chromium.org/10533116/diff/1/chrome/browser/value_store/val...
chrome/browser/value_store/value_store_frontend.cc:83: backend_, db_path));
This is what changed since last round. Moving the PostTask to here (instead of
the Backend constructor) ensures that backend_ holds a reference before
PostTask. Now if PostTask fails and releases its reference, backend_ still keeps
the object alive.

[not at google - send to devlin, 2012-06-12 21:04:00]

lgtm

ah. tricky.