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/tilemodes.cpp b/gm/tilemodes.cpp
index 009ad64..390ce4a 100644
--- a/gm/tilemodes.cpp
+++ b/gm/tilemodes.cpp
@@ -105,7 +105,7 @@
                 str.printf("[%s,%s]", gModeNames[kx], gModeNames[ky]);
 
                 p.setTextAlign(SkPaint::kCenter_Align);
-                canvas->drawText(str.c_str(), str.size(), x + r.width()/2, y, p);
+                canvas->drawString(str, x + r.width()/2, y, p);
 
                 x += r.width() * 4 / 3;
             }
@@ -142,7 +142,7 @@
                     p.setAntiAlias(true);
                     sk_tool_utils::set_portable_typeface(&p);
                     str.printf("%s, %s", gConfigNames[i], gFilterNames[j]);
-                    canvas->drawText(str.c_str(), str.size(), x, y + r.height() * 2 / 3, p);
+                    canvas->drawString(str, x, y + r.height() * 2 / 3, p);
                 }
 
                 y += r.height() * 4 / 3;
@@ -224,7 +224,7 @@
 
         for (size_t kx = 0; kx < SK_ARRAY_COUNT(gModes); kx++) {
             SkString str(gModeNames[kx]);
-            canvas->drawText(str.c_str(), str.size(), x + r.width()/2, y, p);
+            canvas->drawString(str, x + r.width()/2, y, p);
             x += r.width() * 4 / 3;
         }
 
@@ -235,7 +235,7 @@
             x = SkIntToScalar(16) + w;
 
             SkString str(gModeNames[ky]);
-            canvas->drawText(str.c_str(), str.size(), x, y + h/2, p);
+            canvas->drawString(str, x, y + h/2, p);
 
             x += SkIntToScalar(50);
             for (size_t kx = 0; kx < SK_ARRAY_COUNT(gModes); kx++) {