Add an extra GC step to ImageDecoder fuzzer to reduce memory.

When the fuzzer executes with -runs=100, GC doesn't happen quickly
enough to release held resources. We could add a 7 step GC process
like is used elsewhere, but that's very slow. For now, one extra GC
step midway through execution seems to release enough.

R=tguilbert

Fixed: 1223803
Change-Id: I012f26aca34a79e2cff02b18328777c949071f4f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2993584
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Thomas Guilbert <tguilbert@chromium.org>
Auto-Submit: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Thomas Guilbert <tguilbert@chromium.org>
Cr-Commit-Position: refs/heads/master@{#896694}