Include Pepper headers only if ENABLE_PLUGINS is defined

Follow-up to https://codereview.chromium.org/525513003 to fix includes
of Pepper files when ENABLE_PLUGINS isn't defined (Ozone ECS builder).

BUG=none
TBR=markus@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/554693002

Cr-Commit-Position: refs/heads/master@{#293760}

content/zygote/zygote_main_linux.cc

@@ -27,12 +27,10 @@
 #include "build/build_config.h"
 #include "content/common/child_process_sandbox_support_impl_linux.h"
 #include "content/common/font_config_ipc_linux.h"
-#include "content/common/pepper_plugin_list.h"
 #include "content/common/sandbox_linux/sandbox_linux.h"
 #include "content/common/zygote_commands_linux.h"
 #include "content/public/common/content_switches.h"
 #include "content/public/common/main_function_params.h"
-#include "content/public/common/pepper_plugin_info.h"
 #include "content/public/common/sandbox_linux.h"
 #include "content/public/common/zygote_fork_delegate_linux.h"
 #include "content/zygote/zygote_linux.h"
@@ -51,6 +49,11 @@
 #include <openssl/rand.h>
 #endif
 
+#if defined(ENABLE_PLUGINS)
+#include "content/common/pepper_plugin_list.h"
+#include "content/public/common/pepper_plugin_info.h"
+#endif
+
 #if defined(ENABLE_WEBRTC)
 #include "third_party/libjingle/overrides/init_webrtc.h"
 #endif