Clarify subrect semantics for GrTextureAdjuster and handle mip maps correctly.

src/gpu/SkGr.cpp

Index: src/gpu/SkGr.cpp
diff --git a/src/gpu/SkGr.cpp b/src/gpu/SkGr.cpp
index 4685c7699a63941cb12e05a79cb5af1e2eff00c6..469ef5bbe2d2bd149aa3a79875f078824427b536 100644
--- a/src/gpu/SkGr.cpp
+++ b/src/gpu/SkGr.cpp
@@ -346,9 +346,10 @@ private:
         if (fBmp->isVolatile()) {
             return;
         }
-        // The texture subset must represent the whole bitmap. Texture-backed bitmaps don't support
-        // extractSubset(). Therefore, either the bitmap and the teture are the same size or the
-        // subset's dimensions are the bitmap's dimensions.
+        // The content area must represent the whole bitmap. Texture-backed bitmaps don't support
+        // extractSubset(). Therefore, either the bitmap and the texture are the same size or the
+        // content's dimensions are the bitmap's dimensions which is pinned to the upper left
+        // of the texture.
         GrUniqueKey baseKey;
         GrMakeKeyFromImageID(&baseKey, fBmp->getGenerationID(),
                              SkIRect::MakeWH(fBmp->width(), fBmp->height()));