[Extensions] Re-enable a test that's no longer flaky.

The ProxySettingsApiTest.ProxyFixedIndividualIncognitoAlso was
disabled for being flaky. Testing 3000 iterations on the bots
resulted in no flakes.

Bug: 88972
Change-Id: I25f8a9e615894522f0e68512a5c6381121093a84
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2145942Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Commit-Queue: David Bertoni <dbertoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758585}

chrome/browser/extensions/api/proxy/proxy_apitest.cc

@@ -334,9 +334,8 @@ IN_PROC_BROWSER_TEST_F(ProxySettingsApiTest,
 }
 
 // Tests setting values also for incognito mode
-// Test disabled due to http://crbug.com/88972.
 IN_PROC_BROWSER_TEST_F(ProxySettingsApiTest,
-                       DISABLED_ProxyFixedIndividualIncognitoAlso) {
+                       ProxyFixedIndividualIncognitoAlso) {
   ASSERT_TRUE(RunExtensionTestIncognito("proxy/individual_incognito_also")) <<
       message_;
   const Extension* extension = GetSingleLoadedExtension();