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This change moves STYLE_HINT from chrome/ to ui/ to be used as
textfield placeholder text color constant.

The current color used for the placeholder fails the contrast ratio.
For enabled state, this placeholder would have a color similar
to #8F9091 with a contrast ratio of 3.2. For disabled state, the
color would be similar to #B8BBBD with a ratio of 1.93.

With this change, the placeholder will have a text color constant
with a value similar to #5F6368 and a contrast ratio of 6.05.

Bug: 1056950
Change-Id: I66133508491fc79b2e91f1d8cb3013683d088722
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2113796Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Maria Villarreal <mavill@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#753093}