remove deprecated skia transfer mode usage

Skia has removed transfer modes as a class,
and made it an enum.

Update PDFium to match.

R=reed@google.com,dsinclair@chromium.org

Review-Url: https://codereview.chromium.org/2468443002
diff --git a/DEPS b/DEPS
index 51d5b6f..87511c7 100644
--- a/DEPS
+++ b/DEPS
@@ -16,7 +16,7 @@
   'icu_revision': '2341038bf72869a5683a893a2b319a48ffec7f62',
   'instrumented_lib_revision': '45f5814b1543e41ea0be54c771e3840ea52cca4a',
   'pdfium_tests_revision': '06411790a46e6497e5293f921ea9aa9fb2615e41',
-  'skia_revision': 'b55ebf071dce33794f5518dcdc3a9af006285796',
+  'skia_revision': 'a9fd106d0c4dd5449c607add0716db7bbd81bc73',
   'tools_memory_revision': '427f10475e1a8d72424c29d00bf689122b738e5d',
   'trace_event_revision': '6232c13e4edb36c84c61653fdae5a4afb5af9745',
   'v8_revision': 'a964163e6ac4f1124b22ea7e46b8afe26dec02e0',