Fix jank on first show of holding space welcome banner.

Because holding space welcome banner styles are dynamically loaded,
there was some jank when the banner was shown before the dynamic styles
had finished loading.

This CL adds inline styles to hide the welcome banner until they are
removed following load completion of dynamic styles.

Bug: 1170470
Change-Id: I32aa65c5c17505d1e2c2bee61a4e27468c6f881d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2649110


Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#847408}