Rename onGetPixelsEnum back to onGetPixels.

Replace the old signature of onGetPixels (return bool) to return an
enum (Result). Remove onGetPixelsEnum.

Add a define for onGetPixelsEnum to onGetPixels. This is for staging
in Chromium, where some implementations override onGetPixelsEnum.

Add the define in skia_for_chromium_defines. Remove
SK_SUPPORT_LEGACY_IMAGE_GENERATOR_RETURN, which is no longer needed by
Chromium.

BUG=skia:3257

Review URL: https://codereview.chromium.org/939113002
diff --git a/tests/CachedDecodingPixelRefTest.cpp b/tests/CachedDecodingPixelRefTest.cpp
index 51cb7ba..3882026 100644
--- a/tests/CachedDecodingPixelRefTest.cpp
+++ b/tests/CachedDecodingPixelRefTest.cpp
@@ -189,7 +189,7 @@
         return true;
     }
 
-    virtual Result onGetPixelsEnum(const SkImageInfo& info, void* pixels, size_t rowBytes,
+    virtual Result onGetPixels(const SkImageInfo& info, void* pixels, size_t rowBytes,
                                SkPMColor ctable[], int* ctableCount) SK_OVERRIDE {
         REPORTER_ASSERT(fReporter, pixels != NULL);
         REPORTER_ASSERT(fReporter, rowBytes >= info.minRowBytes());