[ARC] [Split this CL] Convert ios/web/webui to ARC.

[ARC] [Split this CL] Convert ios/web/webui to ARC.

Converts ios/web/webui to ARC.

Notable changes:
* ios/web/webui/crw_web_ui_page_builder.mm has a block that had a __block
  reference to itself. Before ARC __block means "do not retain when
  being captured in a block", but with ARC it is captured. So I added an
  additional variable of __weak __block type instead.

* One WeakNSObject in mojo_facade.h could not be removed because it is
included from non-ARC code. WeakNSObject is functional in ARC but is not
preferred. Filed crbug.com/639326 to track future removal of this.

BUG=639327

[stkhapugin, 2016-08-19 16:52:26]

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

[commit-bot: I haz the power, 2016-08-19 16:52:53]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-08-19 17:54:53]

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

[commit-bot: I haz the power, 2016-08-19 17:54:54]

Dry run: Try jobs failed on following builders:
  ios-simulator on master.tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios-simulator/bui...)