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/bench/DisplacementBench.cpp b/bench/DisplacementBench.cpp
index 9edf214..3dd7965 100644
--- a/bench/DisplacementBench.cpp
+++ b/bench/DisplacementBench.cpp
@@ -40,7 +40,7 @@
         paint.setColor(0xFF884422);
         paint.setTextSize(SkIntToScalar(96));
         const char* str = "g";
-        canvas.drawText(str, strlen(str), SkIntToScalar(15), SkIntToScalar(55), paint);
+        canvas.drawString(str, SkIntToScalar(15), SkIntToScalar(55), paint);
     }
 
     void makeCheckerboard() {