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/SampleBitmapRect.cpp b/samplecode/SampleBitmapRect.cpp
index ebfe3bb..ccf2d26 100644
--- a/samplecode/SampleBitmapRect.cpp
+++ b/samplecode/SampleBitmapRect.cpp
@@ -171,7 +171,7 @@
 
     SkCanvas canvas(*bm);
 
-    canvas.drawText(gText, strlen(gText), 0, paint.getTextSize()*4/5, paint);
+    canvas.drawString(gText, 0, paint.getTextSize()*4/5, paint);
 }
 
 class BitmapRectView2 : public SampleView {