A few fixes to make Windows Mojo build work.

R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/685203004

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index 84beb15..a0cdf66 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -858,7 +858,7 @@
     ]
 
     # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
-    #cflags = [ "/wd4267" ]
+    cflags = [ "/wd4267" ]
 
     libs = [
       "cfgmgr32.lib",
@@ -1016,6 +1016,11 @@
     configs -= [ "//build/config/compiler:optimize" ]
     configs += [ "//build/config/compiler:optimize_max" ]
   }
+
+  if (is_win) {
+    # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
+    cflags = [ "/wd4267" ]
+  }
 }
 
 source_set("prefs") {