winaura: Do not create WindowObservers for BrowserPlugin.
WindowObserver is not necessary for BrowserPlugin.
Moreover, if BrowserPlugin navigates to an Interstitial page,
tearing down its embedder causes a crash b/c it tries
to cast the Interstitial's WC's view to WCViewAura, which
actually is a WCViewGuest. The ToRenderWidgetHostViewAura does
not work in this scenario because the RWH's process points to
Interstitial's RenderProcess, whose IsGuest() is false. The
simpler fix hence is to get rid of WindowObserver for
BrowserPlugin.
This will also fix a flaky test on windows:
WebViewTest.InterstitialTeardown.
BUG=316298,297014
Test=On Windows Aura, load an app with a <webview> pointing
to a page that shows an interstitial, e.g. try navigating
to https://test.com. After the interstitial is visible, close
the app. The fix would make the browser not crash.
Review URL: https://codereview.chromium.org/
72283002
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235600 0039d316-1c4b-4281-b951-d872f2087c98