sync: Populate entity counts in about:sync tab

sync: Populate entity counts in about:sync tab

Populates the count of entities per type in about:sync.

Includes C++ code to count and emit the set of deleted and non-deleted
counts, though for space reasons only the non-deleted count is displayed
on the about:sync page.

This calculation is somewhat expensive.  It could be made cheaper by
having an index in the Directory to keep track of which entities belong
to which type, but that doesn't exist at the moment.  For now, we just
avoid calculating these counts unless the about:sync page is open.

BUG=349301
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=274563

[rlarocque, 2014-06-03 00:07:09]

Here's the follow-up to r274348, aka.
https://codereview.chromium.org/290023003/.

I had intended to spend a bit more time making this efficient, but then decided
that modifying the Directory is hard and likely to create bugs.  Since this
won't be running unless about:sync is open, the performance shouldn't really
matter anyway.

Please review.

[maniscalco, 2014-06-03 00:42:40]

LGTM.  I agree about keeping it simple and not trying to make it more efficient.

[rlarocque, 2014-06-03 00:48:01]

The CQ bit was checked by rlarocque@chromium.org

[commit-bot: I haz the power, 2014-06-03 00:48:37]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/rlarocque@chromium.org/302283007/1

[commit-bot: I haz the power, 2014-06-03 05:04:08]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  linux_chromium_chromeos_clang_dbg on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/linux_chromium_chrome...)

[commit-bot: I haz the power, 2014-06-03 06:58:41]

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

[commit-bot: I haz the power, 2014-06-03 06:58:41]

Try jobs failed on following builders:
  linux_chromium_chromeos_clang_dbg on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/linux_chromium_chrome...)

[rlarocque, 2014-06-03 17:08:59]

The CQ bit was checked by rlarocque@chromium.org

[commit-bot: I haz the power, 2014-06-03 17:09:23]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/rlarocque@chromium.org/302283007/1

[commit-bot: I haz the power, 2014-06-03 17:34:34]

Change committed as 274563