Add test_runner hook to dump drag image.

Add test_runner hook to dump drag image.

This is a second attempt at:
https://codereview.chromium.org/904833004/
which caused flakiness in pixel tests and was reverted here:
https://codereview.chromium.org/919273002/

This CL merges the fix from:
https://codereview.chromium.org/918313003/

It also handles the case where dumpDragImage is called but no drag occurs
by dumping a 1x1 empty transparent image.

This is used by:
https://codereview.chromium.org/886323005/

BUG=451759, 458077
Committed: https://crrev.com/656fc851d598be035f826deb1b1b05f227ef5865
Cr-Commit-Position: refs/heads/master@{#316194}

[jackhou1, 2015-02-13 04:57:26]

jackhou@chromium.org changed reviewers:
+ tkent@chromium.org

[jackhou1, 2015-02-13 04:57:27]

tkent, please take a look.

[tkent, 2015-02-13 08:27:27]

The CQ bit was checked by tkent@chromium.org

[tkent, 2015-02-13 08:27:28]

lgtm

[commit-bot: I haz the power, 2015-02-13 08:28:11]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/916893003/20001

[commit-bot: I haz the power, 2015-02-13 09:04:25]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-02-13 09:05:18]

Patchset 2 (id:??) landed as
https://crrev.com/656fc851d598be035f826deb1b1b05f227ef5865
Cr-Commit-Position: refs/heads/master@{#316194}