This patch makes Math.random() behave deterministically when a fixed
random seed is provided. This is done by re-seeding the random number
generator the first time a script requests a random number. Doing this
ensures Math.random() returns the same sequence across page loads and
across iframes.

BUG=chromium:696001

Review-Url: https://codereview.chromium.org/2760393002
Cr-Commit-Position: refs/heads/master@{#44076}