Revert "Enable ChromeSitePerProcessTest.LaunchExternalProtocolFromSubframe on windows."

This reverts commit f89b2b5b.

Reason for revert: test is timing out on windows. See https://crbug.com/1142869

Original change's description:
> Enable ChromeSitePerProcessTest.LaunchExternalProtocolFromSubframe on windows.
>
> Enabling test to see if it is still flaky on Windows.
>
> Bug: 980446
> Change-Id: I6f1b0dce68cce9bd359ebe315ee8782b26888c66
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2498957
> Commit-Queue: James MacLean <wjmaclean@chromium.org>
> Auto-Submit: Aaron Colwell <acolwell@chromium.org>
> Reviewed-by: James MacLean <wjmaclean@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#821188}

TBR=acolwell@chromium.org,wjmaclean@chromium.org

Change-Id: Ie7f5c8d58bc12698c2f22b17a327b2f6f39bcffe
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 980446, 1142869
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2502691Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821273}

chrome/browser/site_isolation/chrome_site_per_process_browsertest.cc

@@ -488,7 +488,8 @@ class MailtoExternalProtocolHandlerDelegate
 // This test is not run on ChromeOS because it registers a custom handler (see
 // ProtocolHandlerRegistry::InstallDefaultsForChromeOS), and handles mailto:
 // navigations before getting to external protocol code.
-#if defined(OS_CHROMEOS)
+// Flaky on Windows. See https://crbug.com/980446
+#if defined(OS_CHROMEOS) || defined(OS_WIN)
 #define MAYBE_LaunchExternalProtocolFromSubframe \
   DISABLED_LaunchExternalProtocolFromSubframe
 #else