Add method for removing bookmark image data.

Add method for removing bookmark image data.

Removes all image data associated with the bookmark, and sets the
user_removed_image flag so the server won't attempt to set a new image
for the bookmark.

BUG=434436
Committed: https://crrev.com/950b89fb23bce3b60e02bcdcccfa7e75c345dc31
Cr-Commit-Position: refs/heads/master@{#314613}

[Rune Fevang, 2015-01-31 00:02:28]

rfevang@chromium.org changed reviewers:
+ noyau@chromium.org

[noyau (Ping after 24h), 2015-02-04 10:45:53]

lgtm

[Rune Fevang, 2015-02-04 19:07:08]

The CQ bit was checked by rfevang@chromium.org

[commit-bot: I haz the power, 2015-02-04 19:08:02]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/891873002/1

[commit-bot: I haz the power, 2015-02-04 20:06:14]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-04 20:08:33]

Patchset 1 (id:??) landed as
https://crrev.com/950b89fb23bce3b60e02bcdcccfa7e75c345dc31
Cr-Commit-Position: refs/heads/master@{#314613}