Enable MSVC warning C4018 for various directories that seem to pass as-is.

gpu/gles2_conform_support/gles2_conform_test.gyp

Index: gpu/gles2_conform_support/gles2_conform_test.gyp
diff --git a/gpu/gles2_conform_support/gles2_conform_test.gyp b/gpu/gles2_conform_support/gles2_conform_test.gyp
index 1f23efa3c3cd0fc507cb27f9f972ad1225f02d40..2abe93f4f946ce807537a87a7ff79973968d43c0 100644
--- a/gpu/gles2_conform_support/gles2_conform_test.gyp
+++ b/gpu/gles2_conform_support/gles2_conform_test.gyp
@@ -8,7 +8,7 @@
     'gles2_conform.gypi',
   ],
   'target_defaults': {
-    'msvs_disabled_warnings': [4005, 4013, 4018, 4101, 4716],
+    'msvs_disabled_warnings': [4005, 4013, 4101, 4716],
      'include_dirs': [
        '<(SHARED_INTERMEDIATE_DIR)/gles2_conform_test_embedded_data',
        '../../third_party/gles2_conform/GTF_ES/glsl/GTF/Source',
@@ -134,7 +134,6 @@
                 'EGLAPIENTRY=',
               ],
               'msvs_disabled_warnings': [
-                4018,  # signed/unsigned mismatch
                 4101,  # unreferenced local variable
                 4715,  # not all control paths return a value
                 4267,  # size_t/unsigned int conversion