Convert enable_plugins to a buildflag header.

content/common/view_messages.h

Index: content/common/view_messages.h
diff --git a/content/common/view_messages.h b/content/common/view_messages.h
index 045f297f75391a56c7d60225378b64a60e64f732..b9d444dbfe04c8058556966598d374301c1dfb1a 100644
--- a/content/common/view_messages.h
+++ b/content/common/view_messages.h
@@ -42,6 +42,7 @@
 #include "media/base/media_log_event.h"
 #include "media/capture/ipc/capture_param_traits.h"
 #include "net/base/network_change_notifier.h"
+#include "ppapi/features/features.h"
 #include "third_party/WebKit/public/platform/WebDisplayMode.h"
 #include "third_party/WebKit/public/platform/WebFloatPoint.h"
 #include "third_party/WebKit/public/platform/WebFloatRect.h"
@@ -553,7 +554,7 @@ IPC_MESSAGE_ROUTED0(ViewMsg_WorkerConnected)
 // for tests.
 IPC_MESSAGE_ROUTED1(ViewMsg_WaitForNextFrameForTests, int /* routing_id */)
 
-#if defined(ENABLE_PLUGINS)
+#if BUILDFLAG(ENABLE_PLUGINS)
 // Reply to ViewHostMsg_OpenChannelToPpapiBroker
 // Tells the renderer that the channel to the broker has been created.
 IPC_MESSAGE_ROUTED2(ViewMsg_PpapiBrokerChannelCreated,
@@ -760,7 +761,7 @@ IPC_MESSAGE_ROUTED3(ViewHostMsg_WebUISend,
                     std::string  /* message */,
                     base::ListValue /* args */)
 
-#if defined(ENABLE_PLUGINS)
+#if BUILDFLAG(ENABLE_PLUGINS)
 // A renderer sends this to the browser process when it wants to access a PPAPI
 // broker. In contrast to FrameHostMsg_OpenChannelToPpapiBroker, this is called
 // for every connection.
@@ -769,7 +770,7 @@ IPC_MESSAGE_ROUTED3(ViewHostMsg_RequestPpapiBrokerPermission,
                     int /* routing_id */,
                     GURL /* document_url */,
                     base::FilePath /* plugin_path */)
-#endif  // defined(ENABLE_PLUGINS)
+#endif  // BUILDFLAG(ENABLE_PLUGINS)
 
 // Send the tooltip text for the current mouse position to the browser.
 IPC_MESSAGE_ROUTED2(ViewHostMsg_SetTooltipText,