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Do this, rather than including common.gypi explicitly in all our gyp files, so that gyp files we use but do not maintain (e.g., third_party/externals/libjpeg/libjpeg.gyp) will include common.gypi too.
Review URL: https://codereview.appspot.com/5820068

git-svn-id: http://skia.googlecode.com/svn/trunk@3411 2bbb7eff-a529-9590-31e7-b0007b416f81