Merge "Add unicode flag to blink::ScriptRegexp, and measure incompatibility for |pattern| content attribute." to M50

"u" flag might cause compatibility issues.
e.g.
  pattern="\a" is same as "a" in BMP mode.
  pattern="\a" is an error in Unicode mode.
  pattern=".." matches to value="🐘" in BMP mode.
  pattern="." matches to value="🐘" in Unicode mode.

BUG=535441,595255

Review URL: https://codereview.chromium.org/1805343002

Cr-Commit-Position: refs/heads/master@{#381468}
(cherry picked from commit f74fd8cda50bb8bcc0c2d5c1fc695f86c7aa581a)

Review URL: https://codereview.chromium.org/1822883002 .

Cr-Commit-Position: refs/branch-heads/2661@{#332}
Cr-Branched-From: ef6f6ae5e4c96622286b563658d5cd62a6cf1197-refs/heads/master@{#378081}
4 files changed