Make blink heap unittests more robust against compiler optimizations.

The MarkTest and DeepTest in out blink heap unittests were a bit too fragile. Basically when compiling on android the compiler decides to optimize out foo in MarkTest and both bar and foo in DeepTest since they were unused. However since our test expect these two objects to be kept alive by our conservative stack scan it failed when they weren't around.
I have added two explicit tests to make sure they are used this should fix the issue.

R=ager@chromium.org, erik.corry@gmail.com, haraken@chromium.org, vegorov@chromium.org, zerny@chromium.org
BUG=340591

Review URL: https://codereview.chromium.org/155843002

git-svn-id: svn://svn.chromium.org/blink/trunk@166509 bbb929c8-8fbe-4397-9dbb-9b2b20218538