Reallow default SkPicture constructor for Chromium

This is breaking the Chromium build in:
external/chromium_org/android_webview/browser/browser_view_renderer.cc

TBR=reed@google.com

Review URL: https://codereview.chromium.org/392593002

gyp/skia_for_chromium_defines.gypi

diff --git a/gyp/skia_for_chromium_defines.gypi b/gyp/skia_for_chromium_defines.gypi
index 93674ef..1e38b2f 100644
--- a/gyp/skia_for_chromium_defines.gypi
+++ b/gyp/skia_for_chromium_defines.gypi
@@ -17,6 +17,7 @@
       'SK_SUPPORT_LEGACY_GETDEVICE',
       'SK_IGNORE_ETC1_SUPPORT',
       'SK_IGNORE_GPU_DITHER',
+      'SK_SUPPORT_LEGACY_DEFAULT_PICTURE_CTOR',
     ],
   },
 }