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This CL fixes a couple of common crashes in CSChromium.

- Entering a URL on the NTP caused a crash because the webState wasn't enabled
when it was navigating. This is fixed by setting webUsageEnabled during
-loadGURLFromLocationBar:transition:

- Tapping some buttons in the toolbar would cause a crash in the tab container
VC's -canPerformAction:withSender:, which was creating an array with objects
that might be nil. This method was only needed to plumb responder chain events
into contained view controllers, which has long since been replaced by 
dispatchers.

Also, per the ObjC style guide, the exception raised in -subviewConstraints is
replaced with a NOTREACHED().

Bug: 
Change-Id: Ic22bc6decfde97530434c95f318b72cf637aaf24
Reviewed-on: https://chromium-review.googlesource.com/594947Reviewed-by: Ed Chin <edchin@chromium.org>
Commit-Queue: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491949}