Adding more warning in core

I figured out the reason for the lack of warnings on Linux that caused the Mac build break. Some warnings were explicitly disabled in "core" on Linux. I re-enabled them.
Review URL: https://codereview.chromium.org/12667012

git-svn-id: http://skia.googlecode.com/svn/trunk@8080 2bbb7eff-a529-9590-31e7-b0007b416f81

gyp/core.gyp

@@ -29,10 +29,6 @@
       'msvs_disabled_warnings': [4244, 4267,4345, 4390, 4554, 4800],
       'conditions': [
         [ 'skia_os in ["linux", "freebsd", "openbsd", "solaris"]', {
-          'cflags': [
-            '-Wno-unused',
-            '-Wno-unused-function',
-          ],
           'link_settings': {
             'libraries': [
               '-lpthread',