FlameChart: improve click to select effectiviness

When user clicks on the chart it usually move the cursor slightly.
We detected this and interpreted the click as drag event.
So I added clickTremor and interpret click as click if the drag delta is small.

BUG=
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/190523003

git-svn-id: svn://svn.chromium.org/blink/trunk@168732 bbb929c8-8fbe-4397-9dbb-9b2b20218538