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/samplecode/SampleApp.cpp b/samplecode/SampleApp.cpp
index 2a4ebdc..df10b1e 100644
--- a/samplecode/SampleApp.cpp
+++ b/samplecode/SampleApp.cpp
@@ -1685,6 +1685,10 @@
 }
 #endif
 
+namespace sk_tool_utils {
+    extern bool gEnablePortableTypeface;
+};
+
 bool SampleWindow::onHandleChar(SkUnichar uni) {
     {
         SkView* view = curr_view(this);
@@ -1738,6 +1742,10 @@
             // only
             toggleFPS();
             break;
+        case 'F':
+            sk_tool_utils::gEnablePortableTypeface ^= true;
+            this->inval(NULL);
+            break;
         case 'g':
             fRequestGrabImage = true;
             this->inval(NULL);