Add test support for setting selected text option for <select> multiple

The test framework returns an empty string as the localized strings are
not supported. This caused the assert to fail on android debug builds,
because of which https://codereview.chromium.org/541693003/ had to be
reverted. This is the chromium side changes needed for the patch to be
restored.

BUG=401506

Review URL: https://codereview.chromium.org/593113003

Cr-Commit-Position: refs/heads/master@{#296648}
1 file changed