ninja: don't write stamp files unless needed
ninja: don't write stamp files unless needed

The majority of 'none' targets already write out a single file
(e.g. foobar.action_rules_copies.stamp) to represent the intermediate
steps taken within the target.  In this common case, use that
file as the representative of the entire output of the target
rather than making an additional stamp file.

(In Chrome's case, there is only one 'none' target where the
above is not the case, and that is arguably a bug.)

Review URL: http://codereview.chromium.org/7827023

git-svn-id: e7e1075985beda50ea81ac4472467b4f6e91fc78@1025 78cadc50-ecff-11dd-a971-7dbc132099af

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