Consolidate SkCodec functions for handling rewind

src/codec/SkWebpCodec.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkWebpCodec.h"
 #include "SkTemplates.h"
 
@@ skipping 135 matching lines @@
     // As stated below, libwebp snaps to even left and top. Make sure top and left are even, so we
     // decode this exact subset.
     // Leave right and bottom unmodified, so we suggest a slightly larger subset than requested.
     desiredSubset->fLeft = (desiredSubset->fLeft >> 1) << 1;
     desiredSubset->fTop = (desiredSubset->fTop >> 1) << 1;
     return true;
 }
 
 SkCodec::Result SkWebpCodec::onGetPixels(const SkImageInfo& dstInfo, void* dst, size_t rowBytes,
                                          const Options& options, SkPMColor*, int*) {
-    switch (this->rewindIfNeeded()) {
-        case kCouldNotRewind_RewindState:
-            return kCouldNotRewind;
-        case kRewound_RewindState:
-            // Rewound to the beginning. Since creation only does a peek, the stream is at the
-            // correct position.
-            break;
-        case kNoRewindNecessary_RewindState:
-            // Already at the right spot for decoding.
-            break;
+    if (!this->rewindIfNeeded()) {
+        return kCouldNotRewind;
     }
 
     if (!conversion_possible(dstInfo, this->getInfo())) {
         return kInvalidConversion;
     }
 
     WebPDecoderConfig config;
     if (0 == WebPInitDecoderConfig(&config)) {
         // ABI mismatch.
         // FIXME: New enum for this?
@@ skipping 77 matching lines @@
                 // Break out of the switch statement. Continue the loop.
                 break;
             default:
                 return kInvalidInput;
         }
     }
 }
 
 SkWebpCodec::SkWebpCodec(const SkImageInfo& info, SkStream* stream)
     : INHERITED(info, stream) {}