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/gm/imagemagnifier.cpp b/gm/imagemagnifier.cpp
index a494f4c..2585ea7 100644
--- a/gm/imagemagnifier.cpp
+++ b/gm/imagemagnifier.cpp
@@ -35,7 +35,7 @@
             paint.setColor(sk_tool_utils::color_to_565(rand.nextBits(24) | 0xFF000000));
             paint.setTextSize(rand.nextRangeScalar(0, 300));
             paint.setAntiAlias(true);
-            canvas->drawText(str, strlen(str), SkIntToScalar(x),
+            canvas->drawString(str, SkIntToScalar(x),
                              SkIntToScalar(y), paint);
         }
         canvas->restore();