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/morphology.cpp b/gm/morphology.cpp
index d1513c4..99603c1 100644
--- a/gm/morphology.cpp
+++ b/gm/morphology.cpp
@@ -36,8 +36,8 @@
         const char* str2 = "XYZ";
         paint.setColor(0xFFFFFFFF);
         paint.setTextSize(64);
-        canvas.drawText(str1, strlen(str1), 10, 55, paint);
-        canvas.drawText(str2, strlen(str2), 10, 110, paint);
+        canvas.drawString(str1, 10, 55, paint);
+        canvas.drawString(str2, 10, 110, paint);
     }
 
     SkISize onISize() override {