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The process of creating MimeHandlerView is:
  * Navigation to resource starts
  * MimeHandlerViewEmbedder is created
  * HTMLPlugInElement::UpdatePlugin is called
  * MimeHandlerViewContainerManager is notified
  * HTMLPlugInElement creates a frame
  * MimeHandlerViewEmbedder observes the new frame and attaches
    GuestView, and destroys itself.

These steps are legitimately disturbed when the embedder is not
authorized to load plugins (e.g., in <webview> when using permissions).
In such cases, the update from HTMLPlugInElement does not reach the
MimeHandlerViewContainerManager and also a plugin frame is not created.
Therefore, MimeHandlerViewEmbedder does not get destroyed.

This CL will change the update process so that even for blocked plugins
the MHVCM is notified (of the failure) which gives it a chance to notify
the browser in turn and clean up the MimeHandlerViewEmbedder.

In line with the change, the test:
  WebViewPluginTest.TestLoadPluginInternalResource
is modified to assert no MimeHandlerViewEmbedders are lying around.

Bug: 967045
Change-Id: Ic63a980883240fe8098a4ee8d5e3ae3b9cc46415
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1629071Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Ehsan Karamad <ekaramad@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Reviewed-by: James MacLean <wjmaclean@chromium.org>
Commit-Queue: Ehsan Karamad <ekaramad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663982}