Wait for parent directory sync before performing server-side copy

Add SyncClient::WaitForUpdateTaskToComplete().
Use the new method from CoypOperation.

BUG=384213
TEST=unit_tests

Review URL: https://codereview.chromium.org/372713004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284877 0039d316-1c4b-4281-b951-d872f2087c98
13 files changed