Replaced ExtractIntegerValueAtIndex with ListValue::GetInteger.

ExtractIntegerValueAtIndex's main piece of functionality was that it
would first try to read the number as a string, and if the failed, it
would fallback to reading it as a double/integer. However, the string
read is not needed, as the javascript side always passes a number.

Bug: 
Change-Id: I189e96865ec834bb8753f0d9f418cb3cd9925d17
Reviewed-on: https://chromium-review.googlesource.com/569848


Commit-Queue: Sky Malice <skym@chromium.org>
Reviewed-by: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488321}