DevTools: improve truncation of numbers in scientific notataion

The patch improves layout of number formatters. If the number
is in scientific notation (i.e. 1.43223e-10), then only mantissa
gets truncated.

BUG=397297
R=vsevik

Review URL: https://codereview.chromium.org/665933004

git-svn-id: svn://svn.chromium.org/blink/trunk@184060 bbb929c8-8fbe-4397-9dbb-9b2b20218538