Only write .in outputs if they have changed.

This reduces the number of targets that have to be built when changing the
status of a runtime-enabled feature dramatically (for content_shell, from
1229 to 44), because these changes only affect the source file and not the
header.

In such cases, ninja notices when the modification time is left alone, and
it can propagate this "clean" (not changed) state to dependent targets.

Review URL: https://codereview.chromium.org/688443007

git-svn-id: svn://svn.chromium.org/blink/trunk@185134 bbb929c8-8fbe-4397-9dbb-9b2b20218538