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/mipmap.cpp b/gm/mipmap.cpp
index 56a78f4..9769cda 100644
--- a/gm/mipmap.cpp
+++ b/gm/mipmap.cpp
@@ -38,7 +38,7 @@
     paint.setTextSize(30);
     SkString str;
     str.printf("scale %g %g", dst.width() / img->width(), dst.height() / img->height());
-//    canvas->drawText(str.c_str(), str.size(), 300, 100, paint);
+//    canvas->drawString(str, 300, 100, paint);
 
     canvas->translate(20, 20);
     for (int i = 0; i < 4; ++i) {