omnibox: experiment with restoring placeholder when caret shows

Shows the "Search Google or type a URL" omnibox placeholder even when
the caret (text edit cursor) is showing / when focused. views::Textfield
works this way, as does <input placeholder="">. Omnibox and the NTP's
"fakebox" are exceptions in this regard and this experiment makes this
more consistent.

R=tommycli@chromium.org
BUG=955585

Change-Id: I23c299c0973f2feb43f7a2be3bd3425a80b06c2d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1582315
Commit-Queue: Dan Beam <dbeam@chromium.org>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654279}