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This CL adds WebTimeActivityProvider which is
BrowserListObserver, a TabStripModelObserver,
AppServiceWrapper::EventListener, and
WebTimeNavigationObserver::EventListener. It is hosted
in AppTimeController.

The instance of this class observes
the TabStripModel of each Browser in BrowserList. It also
observes the WebTimeNavigationObserver instances of each WebContent
hosted in each Browser window. The instance also observes
AppServiceWrapper to learn about the activation/deactivation of
chrome application windows.

WebTimeActivityProvider informs its observers when Chrome becomes
Active, Active with whitelisted content or inactive.

Bug: 1034551
Change-Id: I7d7dd3624ea4fb764c13e35f0ef542759cee8933
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2020522Reviewed-by: Aga Wronska <agawronska@chromium.org>
Commit-Queue: Yilkal Abe <yilkal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737492}