Merge to M45: Fix JS_GetArrayElement to not return an empty handle on success

BUG=chromium:528376
TBR=jochen@chromium.org

Review URL: https://codereview.chromium.org/1337433002 .

(cherry picked from commit 0d0935d3d6603dd7d851b1d5828635230f4c43ca)

Review URL: https://codereview.chromium.org/1364833002 .