Remove an unused enhanced bookmark histogram

Remove an unused enhanced bookmark histogram

EnhancedBookmark.ViewMode was used to detect how many users prefer
ListView over GridView. Since now we have fully migrated to ListView,
this histogram is not used anymore.

BUG=NONE

Committed: https://crrev.com/01940bb4055a8cb54af8d9f8074104a509dadc02
Cr-Commit-Position: refs/heads/master@{#350313}

[Ian Wen, 2015-09-22 23:45:38]

The CQ bit was checked by ianwen@chromium.org to run a CQ dry run

[Ian Wen, 2015-09-22 23:45:49]

ianwen@chromium.org changed reviewers:
+ isherman@chromium.org

[Ian Wen, 2015-09-22 23:45:50]

PTAL

[Ilya Sherman, 2015-09-22 23:47:40]

Please mark the histogram as <obsolete> rather than deleting it from
histograms.xml.  I'm assuming the C++ code has already been removed?

[commit-bot: I haz the power, 2015-09-22 23:47:58]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1363723003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1363723003/1

[Ian Wen, 2015-09-22 23:54:15]

Done. Marked obsolete

[Ilya Sherman, 2015-09-23 00:01:55]

LGTM, thanks.

[Ian Wen, 2015-09-23 00:02:18]

The CQ bit was checked by ianwen@chromium.org

[commit-bot: I haz the power, 2015-09-23 00:04:12]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1363723003/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1363723003/20001

[commit-bot: I haz the power, 2015-09-23 06:06:20]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-09-23 06:07:00]

Patchset 2 (id:??) landed as
https://crrev.com/01940bb4055a8cb54af8d9f8074104a509dadc02
Cr-Commit-Position: refs/heads/master@{#350313}