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/matrixconvolution.cpp b/gm/matrixconvolution.cpp
index 7c1a215..4af12e1 100644
--- a/gm/matrixconvolution.cpp
+++ b/gm/matrixconvolution.cpp
@@ -46,7 +46,7 @@
         paint.setShader(SkGradientShader::MakeLinear(
             pts, fColors, pos, 2, SkShader::kClamp_TileMode));
         const char* str = "e";
-        canvas.drawText(str, strlen(str), SkIntToScalar(-10), SkIntToScalar(80), paint);
+        canvas.drawString(str, SkIntToScalar(-10), SkIntToScalar(80), paint);
 
         // ... tag the data as sRGB, so color-aware devices do gamut adjustment, etc...
         fBitmap.setInfo(SkImageInfo::MakeS32(80, 80, kPremul_SkAlphaType));