[NTP] Show fakebox placeholder text for experiment

If the experiment OmniboxUIExperimentShowPlaceholderWhenCaretShowing
is enabled, do not hide the placeholder text in the fakebox on focus.

Screenshot: https://screenshot.googleplex.com/3GYb3rE1btp.png

Bug: 955585
Change-Id: If1a1b56b402c55f511af749da5a10ed0ef92812b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1584941
Auto-Submit: Kristi Park <kristipark@chromium.org>
Reviewed-by: Kyle Milka <kmilka@chromium.org>
Commit-Queue: Kristi Park <kristipark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654525}