GitLab

 - Add placeholder icon
 - Adjust sizing of percent_bar_chart/realtime_cpu chart
 - Will be updated to adhere to the spec in the future.
 - A follow-up CL will update the labels for the percent_bar_chart
   and the realtime_cpu chart

Screenshot: http://shortn/_4EPGfnWJW6

Bug: 1125150
Test: browser_tests --gtest_filter=DiagnosticsApp*
Change-Id: Id05f75cd46a0b01887788f964c9e19a1843dc900
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2536472
Commit-Queue: Michael Checo <michaelcheco@google.com>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828951}