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/drawatlascolor.cpp b/gm/drawatlascolor.cpp
index de64cab..548641b 100644
--- a/gm/drawatlascolor.cpp
+++ b/gm/drawatlascolor.cpp
@@ -138,7 +138,7 @@
 
         for (int i = 0; i < numModes; ++i) {
             const char* label = SkBlendMode_Name(gModes[i]);
-            canvas->drawText(label, strlen(label),
+            canvas->drawString(label,
                              i*(target.width()+kPad)+kPad, SkIntToScalar(kTextPad),
                              textP);
         }