Disable flaky test ImageWriterWriteFromUrlOperationTest.VerifyFile

This test fails consistently on Mac10.10, Mac10.11, Mac10.12, and
Mac10.13 builders. I can reproduce flakiness on my Linux machine,
so I am disabling this test for now.

TBR=blundell@chromium.org

Bug: 983061
Change-Id: Icb65b137d6b5c80d2c0bb798dc7aa763147cffb3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1698371
Commit-Queue: Alex Oldemeier <aoldemeier@chromium.org>
Reviewed-by: Alex Oldemeier <aoldemeier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676863}

chrome/browser/extensions/api/image_writer_private/write_from_url_operation_unittest.cc

@@ -212,7 +212,7 @@ TEST_F(ImageWriterWriteFromUrlOperationTest, DownloadFile) {
   operation->Cancel();
 }
 
-TEST_F(ImageWriterWriteFromUrlOperationTest, VerifyFile) {
+TEST_F(ImageWriterWriteFromUrlOperationTest, DISABLED_VerifyFile) {
   std::unique_ptr<char[]> data_buffer(new char[kTestFileSize]);
   base::ReadFile(test_utils_.GetImagePath(), data_buffer.get(), kTestFileSize);
   base::MD5Digest expected_digest;