Disable failing WebNavigationApiTest.GetFrame on Chrome OS.

Started failing, apparently, in:
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/linux-chromeos-dbg/8903

TBR=nasko@chromium.org

Bug: 874782
Change-Id: I5c7746682a08fea050bfa711c3f052edf2f7de07
Reviewed-on: https://chromium-review.googlesource.com/c/1337348Reviewed-by: Finnur Thorarinsson <finnur@chromium.org>
Commit-Queue: Finnur Thorarinsson <finnur@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608383}

chrome/browser/extensions/api/web_navigation/web_navigation_apitest.cc

@@ -218,8 +218,8 @@ IN_PROC_BROWSER_TEST_F(WebNavigationApiTest, Api) {
   ASSERT_TRUE(RunExtensionTest("webnavigation/api")) << message_;
 }
 
-// Flaky on Windows. See http://crbug.com/874782
-#if defined(OS_WIN)
+// Flaky on Windows and Chrome OS. See http://crbug.com/874782.
+#if defined(OS_WIN) || defined(OS_CHROMEOS)
 #define MAYBE_GetFrame DISABLED_GetFrame
 #else
 #define MAYBE_GetFrame GetFrame