Chromium no longer needs the SkPicture default constructor

With the landing of the Chromium CL (Remove use of default SkPicture constructor - https://codereview.chromium.org/387343003/) Chromium should no longer require the SK_SUPPORT_LEGACY_DEFAULT_PICTURE_CTOR flag.

R=halcanary@google.com

Author: robertphillips@google.com

Review URL: https://codereview.chromium.org/398463002
diff --git a/gyp/skia_for_chromium_defines.gypi b/gyp/skia_for_chromium_defines.gypi
index 1e38b2f..93674ef 100644
--- a/gyp/skia_for_chromium_defines.gypi
+++ b/gyp/skia_for_chromium_defines.gypi
@@ -17,7 +17,6 @@
       'SK_SUPPORT_LEGACY_GETDEVICE',
       'SK_IGNORE_ETC1_SUPPORT',
       'SK_IGNORE_GPU_DITHER',
-      'SK_SUPPORT_LEGACY_DEFAULT_PICTURE_CTOR',
     ],
   },
 }