Revert "[dpwa] Re-enable the web app file handler tests."

This reverts commit fc8ad75.

Reason for revert: Test still failing (https://ci.chromium.org/ui/p/chromium/builders/ci/mac11-arm64-rel-tests/5409/)

Original change's description:
> [dpwa] Re-enable the web app file handler tests.
>
> Bug: 1270961
> Change-Id: Ie6335b64f1d56634d2da67d93bb36659fdacb0fb
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3572598
> Reviewed-by: Daniel Murphy <dmurph@chromium.org>
> Commit-Queue: Daniel Murphy <dmurph@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#989511}

Bug: 1270961
Change-Id: I6d6b7d6234c01803d0dc0bbadc88078be9eb797c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3575285
Owners-Override: Kristi Park <kristipark@google.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Auto-Submit: Kristi Park <kristipark@google.com>
Commit-Queue: Kristi Park <kristipark@google.com>
Cr-Commit-Position: refs/heads/main@{#989997}

chrome/browser/ui/web_applications/web_app_browsertest.cc

@@ -1913,7 +1913,13 @@ class WebAppBrowserTest_FileHandler : public WebAppBrowserTest {
   base::test::ScopedFeatureList feature_list_;
 };
 
-IN_PROC_BROWSER_TEST_F(WebAppBrowserTest_FileHandler, WebAppFileHandler) {
+// TODO(crbug.com/1270961): Flaky on Win and Mac.
+#if BUILDFLAG(IS_WIN) || BUILDFLAG(IS_MAC)
+#define MAYBE_WebAppFileHandler DISABLED_WebAppFileHandler
+#else
+#define MAYBE_WebAppFileHandler WebAppFileHandler
+#endif
+IN_PROC_BROWSER_TEST_F(WebAppBrowserTest_FileHandler, MAYBE_WebAppFileHandler) {
   os_hooks_suppress_.reset();
   base::ScopedAllowBlockingForTesting allow_blocking;