omnibox: add "show placeholder when caret is showing" experiment to about:flags

R=tommycli@chromium.org

Bug: 955585
Change-Id: If878982be55813cd66c692fd10d30ada5f533474
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1585014
Commit-Queue: Dan Beam <dbeam@chromium.org>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654331}
5 files changed