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This occurs in the NativeViewHostAura::NativeViewDetaching code path where we first remove the
clipping window which is the intermediate parent of the web contents view. The clipping window
is hidden which causes the RWHVA::Hide function to get called which initiates the hiding sequence.
Then the web contents view is reparented to the main view which is still visible. Now the RWHVA::Show
function is called which initiates the show sequence. Eventually the main view is hidden, which then
initiates the hide sequence.

Addressed this with the following changes.
1. WebView::AttachWebContents and WebView::DetachWebContents
   now show and hide the webcontents native view. The
   WebContents is shown and hidden as before in
   WebContentsNativeViewAura::OnWindowVisibilityChanged.

2. Removed the WebContentsNativeViewAura::OnWindowParentChanged function.
   This function was present to show and hide the webcontents if the window was visible.
   This should not be needed with the change in #1 above.

3. Added a new file webview_unittest.cc. This contains the unittest WebViewUnitTest.TestWebViewAttachDetachWebContents
   This is run as part of unit_tests.exe.

BUG=412989
R=sky

Review URL: https://codereview.chromium.org/564553002

Cr-Commit-Position: refs/heads/master@{#294962}