Fix incorrect media_pipeline_integration fuzzer seed_corpus

BUG=751811
R=xhwang@chromium.org,mmoroz@chromium.org
TEST=local build of this fuzzer target produces a non-trivial seed corpus zip

Change-Id: I83d7bc24428200e8581db90714e69d94393d0bcf
Reviewed-on: https://chromium-review.googlesource.com/599013Reviewed-by: Max Moroz <mmoroz@chromium.org>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491623}

media/test/BUILD.gn

@@ -167,5 +167,5 @@ fuzzer_test("media_pipeline_integration_fuzzer") {
     "close_fd_mask=1",
   ]
 
-  seed_corpus = "media/test/data/"
+  seed_corpus = "//media/test/data/"
 }