Add utf-8 flag to win build.

Tell VC++ that the pdfium source files are encoded in UTF-8 so that
it doesn't interpret them as being encoded in the local code page.

Change-Id: Ide74061cad523a873ffa5630781dfc47bf7ae79c
Reviewed-on: https://pdfium-review.googlesource.com/2961
Commit-Queue: dsinclair <dsinclair@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index ecac406..be35522 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -59,6 +59,11 @@
     ]
     ldflags += [ "--coverage" ]
   }
+
+  if (is_win) {
+    # Assume UTF-8 by default to avoid code page dependencies.
+    cflags += [ "/utf-8" ]
+  }
 }
 
 config("pdfium_core_config") {