Only compile the PNG encoder once.

The PNG fuzzer included all of PNGImageDecoder.cpp and effectively
compiled it a second time. Since the fuzzer already depends on the
png encoder it will just be linked together anyway.

This double compilation caused jumbo errors because symbols appeared
twice.

Change-Id: I0ef1502ba90247e34c33055056bdeb808e0dbb89
Reviewed-on: https://chromium-review.googlesource.com/565411Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: bratell at Opera <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#485555}

third_party/WebKit/Source/platform/PngFuzzer.cpp

@@ -24,7 +24,7 @@
 // For more details, see
 // https://chromium.googlesource.com/chromium/src/+/master/testing/libfuzzer/README.md
 
-#include "platform/image-decoders/png/PNGImageDecoder.cpp"
+#include "platform/image-decoders/png/PNGImageDecoder.h"
 #include "platform/testing/BlinkFuzzerTestSupport.h"
 
 namespace blink {