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This CL hooks up InputMethodWinTsf with IME extension. Since ui::TSFBridge
may be null for unit tests, this Cl also adds protection in InputMethodWinTSF.

Bug: 657623
Change-Id: I608ceba376f0fdba537339030d35c11993e16822
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1507234Reviewed-by: Shu Chen <shuchen@chromium.org>
Reviewed-by: Navid Zolghadr <nzolghadr@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Commit-Queue: Siye Liu <siliu@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#641127}