Fix deprecated V8 function use in content/public/test

Fix use of deprecated Object::Get V8 function by instead using the
Context version and checking the return value.

Bug: v8:7283, v8:8562
Change-Id: I5e778f2658ac4ce0482a8d45573e7f257f56349e
Reviewed-on: https://chromium-review.googlesource.com/c/1454548
Auto-Submit: Dan Elphick <delphick@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629310}
1 file changed