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This change puts a prompt string into the omnibox whenever the field is
empty.  Once a character is entered, or when the field later becomes
empty, the hint is shown.

BUG=
R=bshe
TBR=cpu@chromium.org

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: Ib05f091fb339a4c06d83a0502e1d0d1b51b0d277
Reviewed-on: https://chromium-review.googlesource.com/812208Reviewed-by: Biao She <bshe@chromium.org>
Commit-Queue: Christopher Grant <cjgrant@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522317}