DevTools: initial support of box selection on FlameChart

If shift is held when dragging starts on the flame chart rectangle selection box will be drawn for the selected region. Selected range duration will be printed at the left top corner of the rectangle with precision of 0.01 ms.

BUG=407113

Review URL: https://codereview.chromium.org/665423003

git-svn-id: svn://svn.chromium.org/blink/trunk@184194 bbb929c8-8fbe-4397-9dbb-9b2b20218538