Revert "Disable flaky ScreenshotTabObserverTest tests."

This reverts commit b81908f9.

Reason for revert: @DisabledTest: Cannot find symbol, oops.

Original change's description:
> Disable flaky ScreenshotTabObserverTest tests.
> 
> TBR=petewil@chromium.org, jianli@chromium.org, dtrainor@chromium.org, isherman@chromium.org
> NOTRY=True
> 
> Bug: 901856
> Change-Id: I61b14dca18356bb71618150795cddc7cce016574
> Reviewed-on: https://chromium-review.googlesource.com/c/1318911
> Reviewed-by: Henrik Boström <hbos@chromium.org>
> Commit-Queue: Henrik Boström <hbos@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#605659}

TBR=isherman@chromium.org,petewil@chromium.org,dtrainor@chromium.org,jianli@chromium.org,hbos@chromium.org

Change-Id: I6ed97b424d0a649d7f02fd17a0569b6bd64f9069
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 901856
Reviewed-on: https://chromium-review.googlesource.com/c/1318913Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605664}

chrome/android/javatests/src/org/chromium/chrome/browser/feature_engagement/ScreenshotTabObserverTest.java

@@ -57,10 +57,8 @@ public class ScreenshotTabObserverTest {
         });
     }
 
-    // Disabled due to flakiness. https://crbug.com/901856
     @Test
     @SmallTest
-    @DisabledTest
     public void testScreenshotNumberReportingOne() {
         MetricsUtils.HistogramDelta histogramDeltaZeroScreenshots =
                 new MetricsUtils.HistogramDelta("Tab.Screenshot.ScreenshotsPerPage", 0);
@@ -79,10 +77,8 @@ public class ScreenshotTabObserverTest {
                 histogramDeltaTwoScreenshots.getDelta());
     }
 
-    // Disabled due to flakiness. https://crbug.com/901856
     @Test
     @SmallTest
-    @DisabledTest
     public void testScreenshotNumberReportingTwo() {
         MetricsUtils.HistogramDelta histogramDeltaTwoScreenshots =
                 new MetricsUtils.HistogramDelta("Tab.Screenshot.ScreenshotsPerPage", 2);
@@ -93,10 +89,8 @@ public class ScreenshotTabObserverTest {
                 histogramDeltaTwoScreenshots.getDelta());
     }
 
-    // Disabled due to flakiness. https://crbug.com/901856
     @Test
     @SmallTest
-    @DisabledTest
     public void testScreenshotActionReporting() {
         MetricsUtils.HistogramDelta histogramDeltaScreenshotNoAction =
                 new MetricsUtils.HistogramDelta("Tab.Screenshot.Action", 0);