content: Re-enable ChildDiscardableSharedMemoryManagerBrowserTest.LockMemory test.

BUG=
TEST=content_browsertests --gtest_filter=ChildDiscardableSharedMemoryManagerBrowserTest.LockMemory

Review URL: https://codereview.chromium.org/1033553003

Cr-Commit-Position: refs/heads/master@{#322122}

content/child/child_discardable_shared_memory_manager_browsertest.cc

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