FlameChart: out of bounds access to external array.

The scale factor for x was calculated incorrectly because

BUG=319999
R=yurys@chromium.org

Review URL: https://codereview.chromium.org/83733003

git-svn-id: svn://svn.chromium.org/blink/trunk@162669 bbb929c8-8fbe-4397-9dbb-9b2b20218538