Revert of content: Re-enable...

Revert of content: Re-enable ChildDiscardableSharedMemoryManagerBrowserTest.LockMemory test. (patchset #1 id:1 of https://codereview.chromium.org/1033553003/)

Reason for revert:
Still flaky

Original issue's description:
> content: Re-enable ChildDiscardableSharedMemoryManagerBrowserTest.LockMemory test.
>
> BUG=
> TEST=content_browsertests --gtest_filter=ChildDiscardableSharedMemoryManagerBrowserTest.LockMemory
>
> Committed: https://crrev.com/98e563846f88a2df7107e170381a6a5e8bd4454b
> Cr-Commit-Position: refs/heads/master@{#322122}

TBR=avi@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=

Review URL: https://codereview.chromium.org/1038703002

Cr-Commit-Position: refs/heads/master@{#322175}

content/child/child_discardable_shared_memory_manager_browsertest.cc

@@ -49,7 +49,7 @@ class ChildDiscardableSharedMemoryManagerBrowserTest
 };
 
 IN_PROC_BROWSER_TEST_F(ChildDiscardableSharedMemoryManagerBrowserTest,
-                       LockMemory) {
+                       DISABLED_LockMemory) {
   const size_t kSize = 1024 * 1024;  // 1MiB.
 
   NavigateToURL(shell(), GURL(url::kAboutBlankURL));