Re-enable property tree verification for layout tests

Property tree verification was inadvertently disabled by
http://crrev.com/976553002.

This CL re-enables it.

Review URL: https://codereview.chromium.org/1068383002

Cr-Commit-Position: refs/heads/master@{#324409}

content/shell/app/shell_main_delegate.cc

@@ -193,6 +193,8 @@ bool ShellMainDelegate::BasicStartupComplete(int* exit_code) {
     command_line.AppendSwitch(switches::kDisableDelegatedRenderer);
     command_line.AppendSwitch(cc::switches::kCompositeToMailbox);
 
+    command_line.AppendSwitch(cc::switches::kEnablePropertyTreeVerification);
+
     command_line.AppendSwitch(switches::kEnablePreciseMemoryInfo);
 
     command_line.AppendSwitchASCII(switches::kHostResolverRules,