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- Creates a feature flag for Clean Toolbar.
- Creates a ToolbarAdapter class that will be inited when the flag is enabled.
- Adapter conforms to Toolbar protocol, but no-ops all implementation for now.
- Adapter inits a CleanToolbarView controller and plumbs a dispatcher, WebstateList
and ChromeBrowserState.
- Adds a WebStateList parameter to BrowserViewControllerDependencyFactory.
- Adds a convenience viewController getter to AbstractWebToolbar protocol.

CL mainly done by sczs@.

Bug: 784502
Change-Id: Ie0c1fce6062adc123e8226252b827281f96ff081
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Reviewed-on: https://chromium-review.googlesource.com/771713Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517398}