This CL fixes the problem that the platform was set at MockPlatform's
constructor but was not reset.

Bug: v8:8527
Change-Id: I21c3b19320885b1b38999161db7cc1b8f15d798e
Reviewed-on: https://chromium-review.googlesource.com/c/1397821Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58574}