SkColorTable locking serves no purpose anymore.

The only thing the unlock methods were doing was assert their balance.
This removes the unlock methods and renames the lock methods "read".

BUG=skia:

Review URL: https://codereview.chromium.org/719213008
diff --git a/src/images/SkDecodingImageGenerator.cpp b/src/images/SkDecodingImageGenerator.cpp
index cdee7c1..d5c2ba8 100644
--- a/src/images/SkDecodingImageGenerator.cpp
+++ b/src/images/SkDecodingImageGenerator.cpp
@@ -199,8 +199,7 @@
             return false;
         }
         const int count = ctable->count();
-        memcpy(ctableEntries, ctable->lockColors(), count * sizeof(SkPMColor));
-        ctable->unlockColors();
+        memcpy(ctableEntries, ctable->readColors(), count * sizeof(SkPMColor));
         *ctableCount = count;
     }
     return true;