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This change:
1. Disables the options button (next to the 'disable ChromeVox button')
when signed out.
2. Disables certain menu items when signed out.
3. Disabled elements are styled visually and are populated with the
aria-disabled property so their state is announced by ChromeVox.
4. Stores session state in the panel to make this change cleaner.
5. Marks the panel's annotations element as hidden so it's only
accessible when visible.
6. Checks that UserAnnotationHandler.instance is defined before
performing any logic inside its functions. There was a case where
code in the panel triggered a call to UserAnnotationHandler when
its instance was undefined.
7. Ensures that navigation via arrow keys skips disabled menus and
items.

Bug: 1020003
Change-Id: I2b4204970e474054e53bc8425e0e538a9c3de556
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2050789Reviewed-by: David Tseng <dtseng@chromium.org>
Commit-Queue: Akihiro Ota <akihiroota@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747561}