Give build a --write-only-new option.

If that's set, build will only change the mtime on files with new contents.
That allows ninja's restat logic to fire.

BUG=chromium:439182
R=newt@chromium.org

Review URL: https://codereview.chromium.org/757213003

git-svn-id: http://grit-i18n.googlecode.com/svn/trunk@183 7262f16d-afe8-6277-6482-052fa10e57b1
Test for r183. gcl didn't land it, for some reason.

Reviewed at https://codereview.chromium.org/757213003

BUG=chromium:439182

Review URL: https://codereview.chromium.org/766263004

git-svn-id: http://grit-i18n.googlecode.com/svn/trunk@184 7262f16d-afe8-6277-6482-052fa10e57b1