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This patch creates GRC TabSignalObserver for TabManager to get tab scoped
signals from GRC, implements TabDoneLoading signal and eventually sends it to
TabManager::WebContentsData, and changes network idle signal to be persisted.

TBR=japhet@chromium.org
BUG=730098

Change-Id: I5ba9be4365cc79b0d1e6f19e44ec93eeaf0408ee
Reviewed-on: https://chromium-review.googlesource.com/563832
Commit-Queue: lpy <lpy@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Zhen Wang <zhenw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486880}