Fix WebGL roll script for recent ChromeOS-specific DEPS changes.

Add an evaluator for "Str('')".

Tbr: jdarpinian@chromium.org
Tbr: shrekshao@google.com
Bug: 1097235
Change-Id: I9f35c8696215970e95d65dd476f312705fbf59d8
No-Try: True
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2318651


Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791467}