String inputs would end up in the fast-path, crashing because it
expected an array type. Add the fast path explicitly when the source is
a TypedArray, and let everything else fall back to the generic JS
implementation.

Bug: chromium:715971
Change-Id: Ieec28e93279047d403e00ed2676dc1eda193c033
Reviewed-on: https://chromium-review.googlesource.com/493226
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45034}