An innocent-looking change in 'c63e50ed Reland Update V8 DEPS.' exposed
latent bugs in SubStringStub and StringCharFromCodeGenerator.

TEST=mjsunit/string-replace, msjunit/string-split, others....
BUG=

Review URL: https://codereview.chromium.org/1233903003

Cr-Commit-Position: refs/heads/master@{#29731}