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Added a new helper function to determine whether the document has focus,
taking into account whether an iframe or webview element has focus
and whether the document is hidden.

This fixes issues where there were two ChromeVox active indicators showing,
one on the outer frame and one inside the inner frame or webview. It also
fixes issues with other speech-generating events being processed in both
frames at once.

BUG=484904

Review URL: https://codereview.chromium.org/1137403003

Cr-Commit-Position: refs/heads/master@{#329956}