Build Expat on all platforms

* bump version to @android-6.0.1_r55 (2.1.1)

* add as a dependency to the 'xml' target (for upcoming http://crrev.com/2142893006)

* tweak build for Mac, Win:

  - define HAVE_EXPAT_CONFIG_H (same as in Android.mk)
  - use -Wno-missing-field-initializers (same as in Android.mk)
  - suppress MSVS warning 4244 (same as ..cmake/Utilities/cmexpat/expatConfig.h.in)

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2150603004
R=mtklein@google.com

Review-Url: https://codereview.chromium.org/2150603004
diff --git a/DEPS b/DEPS
index a1eddff..3dc7d1c 100644
--- a/DEPS
+++ b/DEPS
@@ -31,7 +31,7 @@
   "third_party/externals/yasm/source/patched-yasm/" : "https://chromium.googlesource.com/chromium/deps/yasm/patched-yasm.git@4671120cd8558ce62ee8672ebf3eb6f5216f909b",
   "third_party/externals/yasm/binaries"             : "https://chromium.googlesource.com/chromium/deps/yasm/binaries.git@52f9b3f4b0aa06da24ef8b123058bb61ee468881",
 
-  "third_party/externals/expat" : "https://android.googlesource.com/platform/external/expat.git@android-5.1.0_r3",
+  "third_party/externals/expat" : "https://android.googlesource.com/platform/external/expat.git@android-6.0.1_r55",
 
 
   # The line below is needed for compiling SkV8Example. Do not delete.