DevTools: Use toISOString instead of toGMTString for dates.

toGMTString is deprecated, and its results are cumbersome.
toISOString output is much more clear and useful.

BUG=377595

Review URL: https://codereview.chromium.org/300103007

git-svn-id: svn://svn.chromium.org/blink/trunk@174934 bbb929c8-8fbe-4397-9dbb-9b2b20218538