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/bmpfilterqualityrepeat.cpp b/gm/bmpfilterqualityrepeat.cpp
index e7ea97d..313fd93 100644
--- a/gm/bmpfilterqualityrepeat.cpp
+++ b/gm/bmpfilterqualityrepeat.cpp
@@ -76,7 +76,7 @@
             bmpPaint.setShader(SkShader::MakeBitmapShader(fBmp, kTM, kTM, &lm));
             bmpPaint.setFilterQuality(kQualities[q].fQuality);
             canvas->drawRect(rect, bmpPaint);
-            canvas->drawText(kQualities[q].fName, strlen(kQualities[q].fName), 20, 40, textPaint);
+            canvas->drawString(kQualities[q].fName, 20, 40, textPaint);
             canvas->translate(250, 0);
         }