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Note: This change is a prerequisite for an upcoming change that will add
extensive interactive UI testing around Flash fullscreen, including the
Fullscreen-Within-Tab UI.

For all interactive_ui_tests, the UI widget being referenced by
VIEW_ID_TAB_CONTAINER was wrong whenever Flash fullscreen mode was
engaged.  This change tags the correct NSView with the identifier, and
gives it some simple focus-passing behavior, similar to
ui::views::WebView, to ensure that tests checking focus and/or
simulating a click on the tab container get the desired behavior.

Because the code changes to TabContentsContainerView are pretty much the
same, this change also addresses bug 421728, which makes clicking in the
"gray area" during Fullscreen-Within-Tab mode a request to give the tab
contents the focus.

BUG=403641,421728

Review URL: https://codereview.chromium.org/642143004

Cr-Commit-Position: refs/heads/master@{#300326}