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/SampleBevel.cpp b/samplecode/SampleBevel.cpp
index fce8885..3f193cf 100644
--- a/samplecode/SampleBevel.cpp
+++ b/samplecode/SampleBevel.cpp
@@ -90,7 +90,7 @@
         labelPaint.setColor(0xFFFFFFFF);
         labelPaint.setTextSize(12.0f);
 
-        canvas->drawText(label.c_str(), label.size(), 0, kLabelHeight - 6.0f, labelPaint);
+        canvas->drawString(label, 0, kLabelHeight - 6.0f, labelPaint);
     }
 
     SkString fName;