Add a pure virtual method to the GarbageCollectedMixin class

Add a pure virtual method to the GarbageCollectedMixin class

It's easy to miss adding USING_GARBAGE_COLLECTED_MIXIN macros to classes
that inherit from GarbageCollectedMixin. To avoid the programming error,
this CL adds a pure virtual method to the GarbageCollectedMixin class.
If we miss the USING_GARBAGE_COLLECTED_MIXIN macro, we'll hit a compile error.

This CL also fixes existing programming errors.

BUG=420515
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=183799

[haraken, 2014-10-16 09:17:48]

haraken@chromium.org changed reviewers:
+ oilpan-reviews@chromium.org

[haraken, 2014-10-16 09:17:48]

PTAL

[kouhei (in TOK), 2014-10-16 09:19:36]

kouhei@chromium.org changed reviewers:
+ kouhei@chromium.org

[kouhei (in TOK), 2014-10-16 09:19:37]

s/USEING/USING/g

[sof, 2014-10-16 09:26:47]

sigbjornf@opera.com changed reviewers:
+ sigbjornf@opera.com

[sof, 2014-10-16 09:26:47]

lgtm, thanks for fixing.

[haraken, 2014-10-16 09:42:32]

The CQ bit was checked by haraken@chromium.org

[commit-bot: I haz the power, 2014-10-16 09:42:51]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/659953002/1

[commit-bot: I haz the power, 2014-10-16 10:23:52]

Committed patchset #1 (id:1) as 183799