Revert "Disable flaky RestoreHistogramTest testWritingHistogramAtStartup."

This reverts commit 434f3432.

Reason for revert:

Findit (https://goo.gl/kROfz5) identified CL at revision 584133 as the
culprit for failures in the build cycles as shown on:
https://findit-for-me.appspot.com/waterfall/culprit?key=ag9zfmZpbmRpdC1mb3ItbWVyRAsSDVdmU3VzcGVjdGVkQ0wiMWNocm9taXVtLzQzNGYzNDMyNTkwODg4Yjg5OGNlODVmZGI5OWVhYmZiYjJkNTlmNzIM

Sample Failed Build: https://ci.chromium.org/buildbot/chromium/android-rel/738

Sample Failed Step: compile

Original change's description:
> Disable flaky RestoreHistogramTest testWritingHistogramAtStartup.
> 
> TBR=tedchoc@chromium.org
> NOTRY=true (disabling a test)
> 
> Bug: 875372
> Change-Id: Ia51964a62207de7300b159e80a3e4816af8f579b
> Reviewed-on: https://chromium-review.googlesource.com/1180165
> Reviewed-by: apacible <apacible@chromium.org>
> Commit-Queue: apacible <apacible@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#584133}

Change-Id: Ib147d1b5da422966072e141e1612b571aaf29587
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 875372
Reviewed-on: https://chromium-review.googlesource.com/1180263
Cr-Commit-Position: refs/heads/master@{#584151}

chrome/android/javatests/src/org/chromium/chrome/browser/RestoreHistogramTest.java

@@ -97,7 +97,6 @@ public class RestoreHistogramTest {
      * @throws ProcessInitException
      */
     @Test
-    @DisabledTest(message = "Test is flaky on. crbug.com/875372")
     @SmallTest
     public void testWritingHistogramAtStartup() throws InterruptedException, ProcessInitException {
         LibraryLoader.getInstance().ensureInitialized(LibraryProcessType.PROCESS_BROWSER);