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/tests/SerializationTest.cpp b/tests/SerializationTest.cpp
index 10aec53..bb49df0 100644
--- a/tests/SerializationTest.cpp
+++ b/tests/SerializationTest.cpp
@@ -438,7 +438,7 @@
     canvas->drawCircle(SkIntToScalar(kBitmapSize/2), SkIntToScalar(kBitmapSize/2), SkIntToScalar(kBitmapSize/3), paint);
     paint.setColor(SK_ColorBLACK);
     paint.setTextSize(SkIntToScalar(kBitmapSize/3));
-    canvas->drawText("Picture", 7, SkIntToScalar(kBitmapSize/2), SkIntToScalar(kBitmapSize/4), paint);
+    canvas->drawString("Picture", SkIntToScalar(kBitmapSize/2), SkIntToScalar(kBitmapSize/4), paint);
 }
 
 DEF_TEST(Serialization, reporter) {