use platform-independent font for gm

Create a custom typeface and scaler to render simple paths the
same on all platforms.

GM tests are modified to explicitly select the custom typeface.

R=reed@google.com, mtklein@google.com, bungeman@google.com
TBR=reed

Author: caryclark@google.com

Review URL: https://codereview.chromium.org/348323003
diff --git a/gyp/utils.gypi b/gyp/utils.gypi
index 761fbf7..5fce7c7 100644
--- a/gyp/utils.gypi
+++ b/gyp/utils.gypi
@@ -121,5 +121,7 @@
         #testing
         '<(skia_src_path)/fonts/SkGScalerContext.cpp',
         '<(skia_src_path)/fonts/SkGScalerContext.h',
+        '<(skia_src_path)/fonts/SkTestScalerContext.cpp',
+        '<(skia_src_path)/fonts/SkTestScalerContext.h',
     ],
 }