Fix conditional disabling of flaky AutomationApiTest.Mixins

Change https://codereview.chromium.org/1014543006 was supposed to disable
AutomationApiTest.Mixins because it is flaky (it fails ~8% of the time on
Windows, and is also flaky on Linux).

However the disabling didn't work because the newly created MAYBE_Mixins
macro is never used. This fixes that by renaming the test.

The line that always fails is:

assertEq(2, input.textSelStart);

The output is:

Actual: 0
Expected: 2

R=ellyjones@chromium.org
TBR=aboxhall@chromium.org
BUG=467921

Review URL: https://codereview.chromium.org/1129733004

Cr-Commit-Position: refs/heads/master@{#330202}

chrome/browser/extensions/api/automation/automation_apitest.cc

@@ -206,7 +206,7 @@ IN_PROC_BROWSER_TEST_F(AutomationApiTest, Find) {
 #else
 #define MAYBE_Mixins Mixins
 #endif
-IN_PROC_BROWSER_TEST_F(AutomationApiTest, Mixins) {
+IN_PROC_BROWSER_TEST_F(AutomationApiTest, MAYBE_Mixins) {
   StartEmbeddedTestServer();
   ASSERT_TRUE(RunExtensionSubtest("automation/tests/tabs", "mixins.html"))
       << message_;