Revert "Re-enable TabCaptureApiTest.ApiTests on Mac."

This reverts commit 0cece6a4.

Reason for revert: The test indeed flakes again.

Original change's description:
> Re-enable TabCaptureApiTest.ApiTests on Mac.
> 
> I have run the test locally multiple times with iteration count of 100
> and 200 without any failures. Re-enabling to see if it will flake on
> the bots or whatever the root cause was is possibly fixed.
> 
> Bug: 764464
> Change-Id: Iedd41ec3b7fa1172766ac32196f3c8dbc2994f9d
> Reviewed-on: https://chromium-review.googlesource.com/716797
> Reviewed-by: Yuri Wiitala <miu@chromium.org>
> Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
> Commit-Queue: Nasko Oskov <nasko@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#508486}

TBR=nasko@chromium.org,miu@chromium.org,alexmos@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 764464
Change-Id: I770164ccc298fdb33cf082fc3aa6aef307c10f49
Reviewed-on: https://chromium-review.googlesource.com/725399Reviewed-by: Martin Šrámek <msramek@chromium.org>
Commit-Queue: Martin Šrámek <msramek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509720}

chrome/browser/extensions/api/tab_capture/tab_capture_apitest.cc

@@ -145,8 +145,14 @@ TEST(TabCaptureCaptureOffscreenTabTest, DetermineInitialSize) {
                 options));
 }
 
+// Flaky on Mac. See https://crbug.com/764464.
+#if defined(OS_MACOSX)
+#define MAYBE_ApiTests DISABLED_ApiTests
+#else
+#define MAYBE_ApiTests ApiTests
+#endif
 // Tests API behaviors, including info queries, and constraints violations.
-IN_PROC_BROWSER_TEST_F(TabCaptureApiTest, ApiTests) {
+IN_PROC_BROWSER_TEST_F(TabCaptureApiTest, MAYBE_ApiTests) {
   AddExtensionToCommandLineWhitelist();
   ASSERT_TRUE(RunExtensionSubtest("tab_capture", "api_tests.html")) << message_;
 }