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According the mock (http://shortn/_WV9xAY7nir), the text "Placeholder"
in the text field and the paper airplane icon in the notification reply
should be transparent white. After inputting text, they should be opaque
white.

However, they were transparent black. After inputting text, they were
opaque black.

This CL fixes the color of them.

BUG=815134
TEST=manual

Change-Id: I68d5ef9373c1cc537e05aa6785c724a55b95b137
Reviewed-on: https://chromium-review.googlesource.com/934482Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Commit-Queue: Megumi Hattori <megumihattori@google.com>
Cr-Commit-Position: refs/heads/master@{#539076}