Split script runs at 'COMMON' blocks except for "white space" character. RTHB merges text runs
if they're connected by 'COMMON' blocks (like symbols and numbers), which might cause the "tofu" issue.
Also Rewrite the line breaker algorithm based on words iteration approach.
However, the root reason for this issue is that partial (sub text-runs) font fallback
algorithm, which will be overhauled in crbug.com/396415.
BUG=473946,448909
TEST=Manually check the website http://th.wikipedia.org/wiki/เยอรมันวิงส์_เที่ยวบินที่_9525 on device Pixel and Peach Pit.
Review URL: https://codereview.chromium.org/
1070223004
Cr-Commit-Position: refs/heads/master@{#333845}