[Extensions] Make extension actions use gfx::Image over gfx::ImageSkia.

[Extensions] Make extension actions use gfx::Image over gfx::ImageSkia.

gfx::Images cache different representations (such as NSImages), whereas
gfx::ImageSkias (as a representation themselves) do not. Prefer to use
gfx::Image in most icon code, so that these representations are cached.

BUG=452971
Committed: https://crrev.com/bf734cdda75f3314996270eef0e924c63cac9743
Cr-Commit-Position: refs/heads/master@{#313768}

[Devlin, 2015-01-29 00:34:34]

Patchset #2 (id:20001) has been deleted

[Devlin, 2015-01-29 01:12:01]

rdevlin.cronin@chromium.org changed reviewers:
+ finnur@chromium.org

[Devlin, 2015-01-29 01:12:02]

Hey Finnur, please take a look when you can.  Follows
https://codereview.chromium.org/885443004/ (ran trybots on first patchset, which
has both patches).

[Finnur, 2015-01-29 09:49:29]

LGTM

[Devlin, 2015-01-29 19:27:52]

The CQ bit was checked by rdevlin.cronin@chromium.org

[commit-bot: I haz the power, 2015-01-29 19:28:53]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/882243002/60001

[commit-bot: I haz the power, 2015-01-29 20:04:25]

Committed patchset #3 (id:60001)

[commit-bot: I haz the power, 2015-01-29 20:06:10]

Patchset 3 (id:??) landed as
https://crrev.com/bf734cdda75f3314996270eef0e924c63cac9743
Cr-Commit-Position: refs/heads/master@{#313768}