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Currently we detect the language only using the surrounding text.
Check user selected text first and fall back to the surrounding text if
the confidence level is no high enough.

Bug: b/172374961
Test: Run existing tests
Change-Id: Ifbcd820828aa7628994b90b756bcd2c9797f30c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2528646Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Jeroen Dhollander <jeroendh@chromium.org>
Commit-Queue: Yue Li <updowndota@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827062}