Mark inspector heap snapshot test as flaky.
Mark inspector heap snapshot test as flaky.

TBR=loislo@chromium.org, yurys@chromium.org
NOTRY=true
BUG=418021

Review URL: https://codereview.chromium.org/609693003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@182742 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs