[iOS] Re-enable EarlGrey test FirstRun.testToggleMetricsOn.

This test appears to have been classified as flaky when in fact the
analysis tools show that there were infrastructure failures for the
ios_chrome_ui_eg2tests_module across various devices.

https://analysis.chromium.org/p/chromium/flake-portal/flakes/occurrences?key=ag9zfmZpbmRpdC1mb3ItbWVyVgsSBUZsYWtlIktjaHJvbWl1bUBpb3NfY2hyb21lX3VpX2VnMnRlc3RzX21vZHVsZUBGaXJzdFJ1blRlc3RDYXNlL3Rlc3RUb2dnbGVNZXRyaWNzT24M

Bug: 1109117
Change-Id: I0ccfebacb5c04c764eb494e9ca43639a9d30005b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2343109Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Nohemi Fernandez <fernandex@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796745}

ios/chrome/browser/ui/first_run/first_run_egtest.mm

@@ -81,8 +81,7 @@ id<GREYMatcher> SkipSigninButton() {
 }
 
 // Toggle the UMA checkbox.
-// TODO(crbug.com/1109117): Test is flaky.
-- (void)DISABLED_testToggleMetricsOn {
+- (void)testToggleMetricsOn {
   [FirstRunAppInterface showFirstRunUI];
 
   id<GREYMatcher> metrics =