Add documention on SkBlitter for runs, and small cleanups.

src/core/SkBlitter.cpp

Index: src/core/SkBlitter.cpp
diff --git a/src/core/SkBlitter.cpp b/src/core/SkBlitter.cpp
index daced55ef2ed33d3fd2a941394468b135980edec..6e10ea5a53cd5587a190b22f624143c63620a9cf 100644
--- a/src/core/SkBlitter.cpp
+++ b/src/core/SkBlitter.cpp
@@ -158,7 +158,7 @@ void SkBlitter::blitMask(const SkMask& mask, const SkIRect& clip) {
                 cy += 1;
             }
         } else {
-            // Bits is calculated as the offset into the mask at the point {cx, cy} therfore, all
+            // Bits is calculated as the offset into the mask at the point {cx, cy} therefore, all
             // addressing into the bit mask is relative to that point. Since this is an address
             // calculated from a arbitrary bit in that byte, calculate the left most bit.
             int bitsLeft = cx - ((cx - maskLeft) & 7);