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This method allows the CastWebView to further delegate this call from
WebContents to the owner of this CastWebView (usually a Cast Activity).
WebBluetooth is disabled for a particular CastWebView unless this
method is overriden by the delegate.

This also adds an implementation of content::BluetoothChooser which
selects devices based on approved access from a remote entity (usually
the Activity host).

BUG=827672

Bug: b/75967216
Test: test app
Change-Id: I7915583012be4abf41b426c789ef40e8714203ad
Reviewed-on: https://chromium-review.googlesource.com/1011417
Commit-Queue: Stephen Lanham <slan@chromium.org>
Reviewed-by: Bailey Forrest <bcf@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550828}