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/tests/ImageFilterTest.cpp b/tests/ImageFilterTest.cpp
index 8a19a22..d884644 100644
--- a/tests/ImageFilterTest.cpp
+++ b/tests/ImageFilterTest.cpp
@@ -738,14 +738,14 @@
             SkScalar ypos = SkIntToScalar(height);
             untiledCanvas.save();
             untiledCanvas.scale(SkIntToScalar(scale), SkIntToScalar(scale));
-            untiledCanvas.drawText(text, strlen(text), 0, ypos, paint);
+            untiledCanvas.drawString(text, 0, ypos, paint);
             untiledCanvas.restore();
             for (int y = 0; y < height; y += tileSize) {
                 for (int x = 0; x < width; x += tileSize) {
                     tiledCanvas.save();
                     tiledCanvas.clipRect(SkRect::Make(SkIRect::MakeXYWH(x, y, tileSize, tileSize)));
                     tiledCanvas.scale(SkIntToScalar(scale), SkIntToScalar(scale));
-                    tiledCanvas.drawText(text, strlen(text), 0, ypos, paint);
+                    tiledCanvas.drawString(text, 0, ypos, paint);
                     tiledCanvas.restore();
                 }
             }