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This is a reland of fa29f6dd

The original change broke this test:
WebXrVrBrowserTestStandard.TestInSessionPermissionNotificationCloseWhileVisible
The reason was that the test assumes that BrowserRenderer will not
change. But, with this change, BrowserRenderer will be re-created
everytime StartOverlay() is called after a StopOverlay(). The fix is
to dynamically query for the BrowserRenderer from
VRBrowserRendererThreadWin.

Original change's description:
> [ui/vr] Display different inheadset notifications
>
> Shows a toast with instructions to exit VR session right after
> session starts.
>
> Also, displays inheadset notifications for previously granted
> permissions when entering immersive presentation, and also when
> they become in-use.
>
> Bug: 913502, 732449, 934056, 937383
> Change-Id: I5d17732da62d07cc7d037160e1914b9fa58f1917
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1527863
> Commit-Queue: Suman Kancherla <sumankancherla@chromium.org>
> Reviewed-by: Bill Orr <billorr@chromium.org>
> Reviewed-by: Klaus Weidner <klausw@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#646117}

Bug: 913502, 732449, 934056, 937383, 947927
Change-Id: Ia06d39fa3ba6c6f47072a35fd242757e4c148f0b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1548606Reviewed-by: Bill Orr <billorr@chromium.org>
Commit-Queue: Suman Kancherla <sumankancherla@chromium.org>
Cr-Commit-Position: refs/heads/master@{#646644}