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/pdf_never_embed.cpp b/gm/pdf_never_embed.cpp
index 4fba40e..ae63432 100644
--- a/gm/pdf_never_embed.cpp
+++ b/gm/pdf_never_embed.cpp
@@ -53,5 +53,5 @@
 
     canvas->scale(1.0, 0.5);
     p.setColor(0xF0000080);
-    canvas->drawText(text, strlen(text), 30, 700, p);
+    canvas->drawString(text, 30, 700, p);
 }