gdata: Make callback parameter mandatory for GetEntryInfo family

gdata: Make callback parameter mandatory for GetEntryInfo family

Looking at callers, it's safe to make the callback parameter mandatory
for GetEntryInfoByResourceId(), GetEntryInfoByPath(),
ReadDirectoryByPath(), as well as helper functions that implement these.
After all these functions are used to get information asynchronously,
hence it does not make sense to call them without the callback parameter.

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

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=151558

[achuithb, 2012-08-14 18:16:04]

lgtm