DevTools: Fix incorrect calculation of y-scale in allocations tracking overview.
DevTools: Fix incorrect calculation of y-scale in allocations tracking overview.

startTime variable was uninitialized before the first invocation
of aggregateAndCall.

R=loislo@chromium.org, yurys@chromium.org

Review URL: https://codereview.chromium.org/16365005

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@151837 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