Remove a clang_warning_flags block from skia.

It's apparently not being used, see
https://codereview.chromium.org/437543007/diff/110001/skia/skia_common.gypi#newcode143

BUG=none
R=hans@chromium.org

Review URL: https://codereview.chromium.org/480463003

Cr-Commit-Position: refs/heads/master@{#290381}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@290381 0039d316-1c4b-4281-b951-d872f2087c98

skia/skia_common.gypi

@@ -130,11 +130,6 @@
           'skia_support_pdf': 1,
         }],
       ],
-      # TODO(scottmg): http://crbug.com/177306
-      'clang_warning_flags': [
-        # Don't warn about string->bool used in asserts.
-        '-Wstring-conversion',
-      ]
     },
     'skia_support_gpu': '<(skia_support_gpu)',
     'skia_support_pdf': '<(skia_support_pdf)',