[storage/browser/fileapi] Replace MessageLoopProxy usage with ThreadTaskRunnerHandle

Replace MessageLoopProxy usage with ThreadTaskRunnerHandle in storage/browser/fileapi module.

MessageLoopProxy is deprecated.
This basically does a search and replace of:

MessageLoopProxy::current() -> ThreadTaskRunnerHandle::Get().

MessageLoopProxy -> SingleThreadTaskRunner

BUG=391045
Committed: https://crrev.com/127a5db6cccb2bc655940a91ecd3ec500703b255
Cr-Commit-Position: refs/heads/master@{#327648}

[Pranay, 2015-04-29 12:10:50]

pranay.kumar@samsung.com changed reviewers:
+ piman@chromium.org

[Pranay, 2015-04-29 12:10:50]

PTAL

Thanks

[piman, 2015-04-29 23:21:31]

lgtm

[Pranay, 2015-04-30 02:40:15]

The CQ bit was checked by pranay.kumar@samsung.com

[commit-bot: I haz the power, 2015-04-30 02:40:25]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1115663002/1

[commit-bot: I haz the power, 2015-04-30 03:43:05]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-30 03:44:00]

Patchset 1 (id:??) landed as
https://crrev.com/127a5db6cccb2bc655940a91ecd3ec500703b255
Cr-Commit-Position: refs/heads/master@{#327648}