Revert mistaken removal of JumpIfBlack prototype.

R=hpayer@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=chromium:561449
LOG=n

Review URL: https://codereview.chromium.org/1544043002

Cr-Commit-Position: refs/heads/master@{#33030}