Reset drag_image_ in web_test_proxy.

Reset drag_image_ in web_test_proxy.

I forgot this in an earlier CL:
https://codereview.chromium.org/904833004/

It needs to be reset so that WebTestProxy is in a clean state for
subsequent tests.

BUG=458077

[jackhou1, 2015-02-13 00:13:17]

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

[jackhou1, 2015-02-13 00:13:18]

tkent, could you please take a look?

[tkent, 2015-02-13 01:26:43]

The CQ bit was checked by tkent@chromium.org

[tkent, 2015-02-13 01:26:44]

lgtm

[commit-bot: I haz the power, 2015-02-13 01:27:30]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/918313003/1

[commit-bot: I haz the power, 2015-02-13 02:00:33]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-02-13 02:00:35]

Try jobs failed on following builders:
  android_arm64_dbg_recipe on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/android_arm64_d...)

[tkent, 2015-02-13 02:02:01]

The CQ bit was checked by tkent@chromium.org

[commit-bot: I haz the power, 2015-02-13 02:02:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/918313003/1

[commit-bot: I haz the power, 2015-02-13 02:29:07]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-02-13 02:29:07]

Try jobs failed on following builders:
  android_arm64_dbg_recipe on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/android_arm64_d...)

[tkent, 2015-02-13 02:30:09]

Hmm, this doesn't pass CQ.  Let me stop CQ for this, and revert
https://codereview.chromium.org/904833004/ .

[jackhou1, 2015-02-13 04:54:19]

On 2015/02/13 02:30:09, tkent wrote:
> Hmm, this doesn't pass CQ.  Let me stop CQ for this, and revert
> https://codereview.chromium.org/904833004/ .

Changes merged into https://codereview.chromium.org/916893003/