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This patch introduces a EmbedderHidden() method that enables bubble
WebUIController subclasses to take appropriate action when their
embedder is no longer visible.

TabSearchUI overrides this to destroy the |page_handler_| when its
embedding bubble is no longer visible to prevent background updates.

Cleaned up use of member variables in WebUIBubbleManagerBase and
WebUIBubbleManager<T>.

Bug: 1099917, 1166967
Change-Id: I374dd74b5a1c3d37a53e7213e64c096adef35bda
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2626114Reviewed-by: Tommy Li <tommycli@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Reviewed-by: Yuheng Huang <yuhengh@chromium.org>
Commit-Queue: Thomas Lukaszewicz <tluk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844343}