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/SkImageDecoder_libwebp.cpp b/src/images/SkImageDecoder_libwebp.cpp
index 8bf15c9..2a7bb0f 100644
--- a/src/images/SkImageDecoder_libwebp.cpp
+++ b/src/images/SkImageDecoder_libwebp.cpp
@@ -621,7 +621,6 @@
     }
 
     SkAutoLockPixels alp(bm);
-    SkAutoLockColors ctLocker;
     if (NULL == bm.getPixels()) {
         return false;
     }
@@ -638,7 +637,7 @@
     pic.writer = stream_writer;
     pic.custom_ptr = (void*)stream;
 
-    const SkPMColor* colors = ctLocker.lockColors(bm);
+    const SkPMColor* colors = bm.getColorTable() ? bm.getColorTable()->readColors() : NULL;
     const uint8_t* src = (uint8_t*)bm.getPixels();
     const int rgbStride = pic.width * bpp;