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Add the full name of current IME as the tooltip and the accessible name
for the IME indicator icon. This change also revises the TrayItemView
by extending the views::Label class. We are using some text like "US"
or "あ" (the first hiragana alphabet of Japanese language) as the icon,
but those should not be read literally like a normal label.

Bug: 888072
Change-Id: I2295a15f6891731487f17dac29c8b9886b6c0b4b
Reviewed-on: https://chromium-review.googlesource.com/c/1317299Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Commit-Queue: Tatsuhisa Yamaguchi <yamaguchi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605279}