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[Media Router] Factor out calls to EventPageRequestManager out of MediaRouterMojoImpl into its own class

On desktop, the calls to the MRP are queued with EventPageRequestManager until
the Mojo connections with the extension are established. This CL factors out the
calls to EventPageRequestManager into MediaRouterDesktop, a subclass of
MediaRouterMojoImpl, so that MediaRouterMojoImpl is not aware of extension-
related details.

Changes to unit tests:
- Move tests from MRMojoImplTest to MRMojoTest so that they can shared with
  MRDesktopTest
- Merge MRMojoExtensionTest into MRDesktopTest

Bug: 737320
Change-Id: I8271932b0418af2490a2bbea49e444120d9e2aa6
Reviewed-on: https://chromium-review.googlesource.com/572501Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Derek Cheng <imcheng@chromium.org>
Commit-Queue: Takumi Fujimoto <takumif@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491861}