Disable flaky test for ThumbnailDiskStorageTest

Failing test:
ThumbnailDiskStorageTest#testRetrieveThumbnailShouldMakeEntryMostRecent

TBR=qinmin@chromium.org
BUG=786212
NOTRY=true

Change-Id: I3a52e5017406efe413f7cbc0f3e37a9762914653
Reviewed-on: https://chromium-review.googlesource.com/776129Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517228}

chrome/android/javatests/src/org/chromium/chrome/browser/widget/ThumbnailDiskStorageTest.java

@@ -213,6 +213,7 @@ public class ThumbnailDiskStorageTest {
     /**
      * Verify that retrieveThumbnail makes the called entry the most recent entry in cache.
      */
+    @DisabledTest(message = "crbug.com/786212")
     @Test
     @SmallTest
     public void testRetrieveThumbnailShouldMakeEntryMostRecent() throws Throwable {