Make SkTDArray accessors const-friendly.

This change creates const and non-const versions of SkTDArray::begin(), end(), operator[]() and getAt(). This will keep us from inadvertently changing a const SkTDArray, which the previous signatures allowed.

Review URL: https://chromiumcodereview.appspot.com/12315131

git-svn-id: http://skia.googlecode.com/svn/trunk@7902 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/images/SkImageDecoder_libbmp.cpp b/src/images/SkImageDecoder_libbmp.cpp
index 2cbdcc3..3aa834a 100644
--- a/src/images/SkImageDecoder_libbmp.cpp
+++ b/src/images/SkImageDecoder_libbmp.cpp
@@ -68,7 +68,7 @@
 
     int width() const { return fWidth; }
     int height() const { return fHeight; }
-    uint8_t* rgb() const { return fRGB.begin(); }
+    const uint8_t* rgb() const { return fRGB.begin(); }
 
 private:
     SkTDArray<uint8_t> fRGB;