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/SurfaceTest.cpp b/tests/SurfaceTest.cpp
index 58ced7e..aa351ed 100644
--- a/tests/SurfaceTest.cpp
+++ b/tests/SurfaceTest.cpp
@@ -332,7 +332,7 @@
     EXPECT_COPY_ON_WRITE(drawBitmap(testBitmap, 0, 0))
     EXPECT_COPY_ON_WRITE(drawBitmapRect(testBitmap, testRect, nullptr))
     EXPECT_COPY_ON_WRITE(drawBitmapNine(testBitmap, testIRect, testRect, nullptr))
-    EXPECT_COPY_ON_WRITE(drawText(testText.c_str(), testText.size(), 0, 1, testPaint))
+    EXPECT_COPY_ON_WRITE(drawString(testText, 0, 1, testPaint))
     EXPECT_COPY_ON_WRITE(drawPosText(testText.c_str(), testText.size(), testPoints2, \
         testPaint))
     EXPECT_COPY_ON_WRITE(drawTextOnPath(testText.c_str(), testText.size(), testPath, nullptr, \