hide SkBitmap::Config entirely (behind a flag)

patch from issue 339463002

TBR=

I think the NoGPU failure is unrelated, so ignoring
NOTRY=True

Author: reed@google.com

Review URL: https://codereview.chromium.org/340533002
diff --git a/gyp/skia_for_chromium_defines.gypi b/gyp/skia_for_chromium_defines.gypi
index cb75613..4f4b8d0 100644
--- a/gyp/skia_for_chromium_defines.gypi
+++ b/gyp/skia_for_chromium_defines.gypi
@@ -14,6 +14,7 @@
     #
     'skia_for_chromium_defines': [
       'SK_SUPPORT_LEGACY_GETTOPDEVICE',
+      'SK_SUPPORT_LEGACY_BITMAP_CONFIG',
       'SK_SUPPORT_LEGACY_DEVICE_VIRTUAL_ISOPAQUE',
       'SK_SUPPORT_LEGACY_N32_NAME',
       'SK_SUPPORT_LEGACY_SETCONFIG',