gdata: Make callback parameters mandatory for CloseFile functions

gdata: Make callback parameters mandatory for CloseFile functions

There was a place where CloseFile() is called with a null callback, but
with the caller fixed, it's safe to make the callback parameter mandatory.

Along the way, removed unused |file_path| parameter from
DataFileSystem::OnGetEntryCompleteForCloseFile()

BUG=126634
TEST=out/Release/unit_tests --gtest_filter=GData*

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=151366

[achuithb, 2012-08-13 21:24:50]

lgtm