add drawString helper to canvas

Many tests and examples use drawText with
a guess of how long the text is in bytes,
or a call to strlen(). Add a helper to
SkCanvas to simplify these examples.

Add another helper for SkString.

R=reed@google.com

Change-Id: I0204a31e938f065606f08ee7cd9a6b36db791ee2
Reviewed-on: https://skia-review.googlesource.com/13642
Commit-Queue: Cary Clark <caryclark@google.com>
Reviewed-by: Cary Clark <caryclark@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Cary Clark <caryclark@skia.org>
diff --git a/samplecode/ClockFaceView.cpp b/samplecode/ClockFaceView.cpp
index 2721a5b..959ac50 100644
--- a/samplecode/ClockFaceView.cpp
+++ b/samplecode/ClockFaceView.cpp
@@ -235,7 +235,7 @@
         paint.setTypeface(fFace);
 
         apply_shader(&paint, SkScalarToFloat(fInterp));
-        canvas->drawText(str.c_str(), str.size(), x, y, paint);
+        canvas->drawString(str, x, y, paint);
 
     //    drawdots(canvas, paint);