drive: Add ResourceMetadataStorage::Iterator::GetCacheEntry()

drive: Add ResourceMetadataStorage::Iterator::GetCacheEntry()

Seeking a nearby entry can be about 20x faster than looking up the whole DB.

BUG=262323
TEST=unit_tests
R=kinaba@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=213750

[hashimoto, 2013-07-25 09:26:58]

Please review.
Below is shown how the 'OFFLINE' case performance gets improved with this
change.

Before:
2013-07-25T18:24:34.492: fileBrowserPrivate.searchDriveMetadata[644] called.
(types: 'SHARED_WITH_ME', maxResults: '500')
2013-07-25T18:24:34.550: fileBrowserPrivate.searchDriveMetadata[644] succeeded.
(elapsed time: 57ms)
2013-07-25T18:24:36.075: fileBrowserPrivate.searchDriveMetadata[650] called.
(types: 'EXCLUDE_DIRECTORIES', maxResults: '500')
2013-07-25T18:24:36.450: fileBrowserPrivate.searchDriveMetadata[650] succeeded.
(elapsed time: 374ms)
2013-07-25T18:24:38.994: fileBrowserPrivate.searchDriveMetadata[656] called.
(types: 'OFFLINE', maxResults: '500')
2013-07-25T18:24:41.873: fileBrowserPrivate.searchDriveMetadata[656] succeeded.
(elapsed time: 2879ms)

After:
2013-07-25T18:22:39.600: fileBrowserPrivate.searchDriveMetadata[734] called.
(types: 'SHARED_WITH_ME', maxResults: '500')
2013-07-25T18:22:39.660: fileBrowserPrivate.searchDriveMetadata[734] succeeded.
(elapsed time: 59ms)
2013-07-25T18:22:40.455: fileBrowserPrivate.searchDriveMetadata[741] called.
(types: 'EXCLUDE_DIRECTORIES', maxResults: '500')
2013-07-25T18:22:40.830: fileBrowserPrivate.searchDriveMetadata[741] succeeded.
(elapsed time: 374ms)
2013-07-25T18:22:42.116: fileBrowserPrivate.searchDriveMetadata[1230] called.
(types: 'OFFLINE', maxResults: '500')
2013-07-25T18:22:42.569: fileBrowserPrivate.searchDriveMetadata[1230] succeeded.
(elapsed time: 453ms)

[kinaba, 2013-07-26 01:30:32]

Great! lgtm

[hashimoto, 2013-07-26 03:25:25]

Committed patchset #2 manually as r213750 (presubmit successful).