Fix BiDi directionality for surrogate pair characters

This patch fixes BiDi directionality for non-BMP (surrogate pair)
characters.

LayoutText::codepointAt was changed to use ICU U16_GET function that
can compute even when the index is at the mid of a surrogate pair.

Rebaseline incorrect expected for svg/text/surrogate-pair-queries.html

BUG=488904

Review-Url: https://codereview.chromium.org/2473863003
Cr-Commit-Position: refs/heads/master@{#429790}
5 files changed