Add a way to inject extra cflags in release builds.

(I intend to use this as a quickie way to add
-g to Release builds so I can see symbols.
Is there a better way?)

Review URL: http://codereview.chromium.org/159621

git-svn-id: http://src.chromium.org/svn/trunk/src/build@22064 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -333,6 +333,7 @@
           'Release': {
             'variables': {
               'release_optimize%': '2',
+              'release_extra_cflags%': '',
             },
             'cflags': [
               '-O<(release_optimize)',
@@ -343,6 +344,7 @@
               # can be removed at link time with --gc-sections.
               '-fdata-sections',
               '-ffunction-sections',
+              '<(release_extra_cflags)',
             ],
           },
         },