DevTools: Add more decimal digits to the FlameChart scale

On extreme FlameChart zoom levels several adjacent grid labels might
become identical. The patch adds necessary decimal digits to make them
distinct.

Review URL: https://codereview.chromium.org/25303002

git-svn-id: svn://svn.chromium.org/blink/trunk@158603 bbb929c8-8fbe-4397-9dbb-9b2b20218538