Revert "Disabling Reader Mode test which flakes in M84."

This reverts commit f26312b0.

Reason for revert: This was only landed to be cherry-picked to 
M84. Reverting in M85 as the test doesn't flake in M85.

Original change's description:
> Disabling Reader Mode test which flakes in M84.
> 
> Flakes are only occurring in M84 because a flake fix landed
> after the 84 branch: crrev.com/c/2209281.
> 
> This change will be cherry-picked to M84 and then reverted in M85.
> 
> Bug: 1064776
> Change-Id: I6518458ac57f268b8c762097877b077e8852d833
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2250159
> Reviewed-by: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
> Commit-Queue: Katie Dektar <katie@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#779885}

TBR=wychen@chromium.org,katie@chromium.org

Change-Id: I406b4fcd74b5af99cbc73f6aef9138b25ddfc7fd
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1064776
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2252694Reviewed-by: Katie Dektar <katie@chromium.org>
Commit-Queue: Katie Dektar <katie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779928}

chrome/browser/dom_distiller/dom_distiller_viewer_source_browsertest.cc

@@ -624,9 +624,7 @@ IN_PROC_BROWSER_TEST_F(DomDistillerViewerSourceBrowserTest, PrefPersist) {
   ASSERT_FLOAT_EQ(kScale, fontSize / oldFontSize);
 }
 
-// Disabled for flakes, crbug.com/1064776.
-IN_PROC_BROWSER_TEST_F(DomDistillerViewerSourceBrowserTest,
-                       DISABLED_UISetsPrefs) {
+IN_PROC_BROWSER_TEST_F(DomDistillerViewerSourceBrowserTest, UISetsPrefs) {
   content::WebContents* contents =
       browser()->tab_strip_model()->GetActiveWebContents();