Make SK_BUILD_FOR_WIN32 auto-detection work.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2205173003

No public API changes
TBR=reed@google.com

Review-Url: https://codereview.chromium.org/2205173003
diff --git a/gyp/common_conditions.gypi b/gyp/common_conditions.gypi
index 65ef9ed..f32d116 100644
--- a/gyp/common_conditions.gypi
+++ b/gyp/common_conditions.gypi
@@ -65,7 +65,6 @@
     [ 'skia_os == "win"',
       {
         'defines': [
-          'SK_BUILD_FOR_WIN32',
           '_CRT_SECURE_NO_WARNINGS',
           'GR_GL_FUNCTION_TYPE=__stdcall',
           '_HAS_EXCEPTIONS=0',
diff --git a/include/core/SkPreConfig.h b/include/core/SkPreConfig.h
index f993920..806ec8a 100644
--- a/include/core/SkPreConfig.h
+++ b/include/core/SkPreConfig.h
@@ -24,7 +24,7 @@
         #include "TargetConditionals.h"
     #endif
 
-    #if defined(WIN32) || defined(__SYMBIAN32__)
+    #if defined(_WIN32) || defined(__SYMBIAN32__)
         #define SK_BUILD_FOR_WIN32
     #elif defined(ANDROID)
         #define SK_BUILD_FOR_ANDROID