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/SampleIdentityScale.cpp b/samplecode/SampleIdentityScale.cpp
index 95970e7..5fbba69 100644
--- a/samplecode/SampleIdentityScale.cpp
+++ b/samplecode/SampleIdentityScale.cpp
@@ -71,7 +71,7 @@
         }
         canvas->drawBitmap( fBM, 100, 100, &paint );
         canvas->restore();
-        canvas->drawText( text, strlen(text), 100, 400, paint );
+        canvas->drawString(text, 100, 400, paint );
         this->inval(nullptr);
     }