Revert of Disable flaky test (patchset #2 id:20001 of https://codereview.chromium.org/2564213002/ )

Reason for revert:
Test is no longer flaky.

Original issue's description:
> Disable flaky test
>
> ResourcePrefetchPredictorBrowserTest.LearningJavascriptAppendChild was
> detected to be flaky by chromium-try-flakes.
>
> TBR=lizeb@chromium.org
>
> BUG=673028
>
> Committed: https://crrev.com/8ae700640b7237af3acc10025827497be8376aab
> Cr-Commit-Position: refs/heads/master@{#437907}

TBR=asargent@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=673028

Review-Url: https://codereview.chromium.org/2575453002
Cr-Commit-Position: refs/heads/master@{#438097}

chrome/browser/predictors/resource_prefetch_predictor_browsertest.cc

@@ -468,9 +468,8 @@ IN_PROC_BROWSER_TEST_F(ResourcePrefetchPredictorBrowserTest,
   NavigateToURLAndCheckSubresources(GetURL(kHtmlDocumentWritePath));
 }
 
-// Disabled due to flakiness (crbug.com/673028).
 IN_PROC_BROWSER_TEST_F(ResourcePrefetchPredictorBrowserTest,
-                       DISABLED_LearningJavascriptAppendChild) {
+                       LearningJavascriptAppendChild) {
   auto externalScript =
       AddExternalResource(GetURL(kScriptAppendChildPath),
                           content::RESOURCE_TYPE_SCRIPT, net::MEDIUM);