Added --force-ui-direction flag for developers to force LTR or RTL.

Setting this flag to ltr or rtl will force the system-wide text
direction to the chosen direction. This allows developers to test RTL
layouts without having to actually switch their language to Hebrew or
Arabic.

BUG=558208

Review URL: https://codereview.chromium.org/1458043003

Cr-Commit-Position: refs/heads/master@{#363135}
8 files changed