Exclude failing DevToolsManagerTest.TestObserver on memory bots

BUG=418258
TBR=dgozman@chromium.org

Review URL: https://codereview.chromium.org/616633003

Cr-Commit-Position: refs/heads/master@{#297283}

tools/valgrind/gtest_exclude/content_unittests.gtest.txt

@@ -7,3 +7,6 @@ WebLayerImplFixedBoundsTest.CompareToWebLayerImplSimple
 
 # Flaky http://crbug.com/413775
 ServiceWorkerCacheStorageManagerTests/ServiceWorkerCacheStorageManagerTestP.RecreateCacheOnDemand/1
+
+# http://crbug.com/418258
+DevToolsManagerTest.TestObserver