Change back the UI font size to 12px/75%

Change back the UI font size to 12px/75%

Roboto's hinted height got back to Roboto v1 value.
The font size (web UI and native UI) got increased to make up
for Roboto v2's height reduction by 1 px at @12ppem (and other sizes).

With Roboto v2's height back to the previous value, we need to set the
UI font size back to the previous value.

Going back to 12px also gives more room for tall scripts such as
Thai, Devanagari, Vietnamese in UI areas like Omnibox.

See https://chromium-review.googlesource.com/293026 for a CrOS CL
to update Roboto to the latest hinted version.

I also updated the expected value for ShelfLayoutInUnifiedDesktop.

BUG=462015
TEST=Manual tests in WebUI and native UI.

Review URL: https://codereview.chromium.org/1282953002

Cr-Commit-Position: refs/heads/master@{#345981}