Disable flaky StreamsPrivateAPI tests on ChromeOS.

These two tests are flaky on the Linux ChromiumOS bot:
https://uberchromegw.corp.google.com/i/chromium.chromiumos/waterfall?show=Linux%20ChromiumOS%20Tests%20(1)

TBR=zork@chromium.org

Bug: 746526
Change-Id: Id2bcf3b9ece0fb25efeff129569d8e96e598ba52
Reviewed-on: https://chromium-review.googlesource.com/578319Reviewed-by: Scott Little <sclittle@chromium.org>
Commit-Queue: Scott Little <sclittle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487954}

chrome/browser/extensions/api/streams_private/streams_private_apitest.cc

@@ -322,10 +322,17 @@ IN_PROC_BROWSER_TEST_F(StreamsPrivateApiTest, NavigateCrossSite) {
   EXPECT_TRUE(catcher.GetNextResult());
 }
 
+// Flaky on ChromeOS: http://crbug.com/746526.
+#if defined(OS_CHROMEOS)
+#define MAYBE_NavigateToAnAttachment DISABLED_NavigateToAnAttachment
+#else
+#define MAYBE_NavigateToAnAttachment NavigateToAnAttachment
+#endif
+
 // Tests that navigation to an attachment starts a download, even if there is an
 // extension with a file browser handler that can handle the attachment's MIME
 // type.
-IN_PROC_BROWSER_TEST_F(StreamsPrivateApiTest, NavigateToAnAttachment) {
+IN_PROC_BROWSER_TEST_F(StreamsPrivateApiTest, MAYBE_NavigateToAnAttachment) {
   InitializeDownloadSettings();
 
   ASSERT_TRUE(LoadTestExtension()) << message_;
@@ -360,10 +367,17 @@ IN_PROC_BROWSER_TEST_F(StreamsPrivateApiTest, NavigateToAnAttachment) {
   EXPECT_TRUE(catcher.GetNextResult());
 }
 
+// Flaky on ChromeOS: http://crbug.com/746526.
+#if defined(OS_CHROMEOS)
+#define MAYBE_DirectDownload DISABLED_DirectDownload
+#else
+#define MAYBE_DirectDownload DirectDownload
+#endif
+
 // Tests that direct download requests don't get intercepted by
 // StreamsResourceThrottle, even if there is an extension with a file
 // browser handler that can handle the download's MIME type.
-IN_PROC_BROWSER_TEST_F(StreamsPrivateApiTest, DirectDownload) {
+IN_PROC_BROWSER_TEST_F(StreamsPrivateApiTest, MAYBE_DirectDownload) {
   InitializeDownloadSettings();
 
   ASSERT_TRUE(LoadTestExtension()) << message_;