[SyncFS] Wire up MetadataDatabaseIndex to MetadataDatabase

Use MetadataDatabaseIndex as the separate index management class in MetadataDatabase.

BUG=344769
NOTRY=true
TEST=unit_tests --gtest_filter='DriveBackendSyncTest.*:MetadataDatabaseTest.*'

Review URL: https://codereview.chromium.org/177233005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255340 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255340
NaCl SDK: Remove more unused variables in examples

PNaCl is being upgraded to Clang 3.4, which now warns about unused
declarations inside anonymous namespaces.

BUG=https://code.google.com/p/nativeclient/issues/detail?id=3757
TEST=none
R=binji@chromium.org

Review URL: https://codereview.chromium.org/180953007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255341 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255341