Implements Drag and Drop for Ozone/X11

It introduces X11DragSource and X11DragContext to handle DnD
on Ozone/X11. They are from DesktopDragDropClientAuraX11 which
includes implementation for platform layer.

X11DragSource handles event and deliver the data at a server side
and X11DragContext handles events and read the data at a client
side.

It splits OSExchangeDataProviderAuraX11 to OSExchangeDataProviderAuraX11
and OSExchangeDataProviderAuraX11Base to resue it.
OSExchangeDataProviderAuraX11 keeps only platform event specific part.

ui/base/BUILD.gn

@@ -453,9 +453,12 @@ jumbo_component("base") {
     deps += [ "//ui/events" ]
   }
 
+  if (use_x11 || ozone_platform_x11) {
+    configs += [ "//build/config/linux:x11" ]
+  }
+
   if (use_x11) {
     public_deps += [ "//ui/base/x" ]
-    configs += [ "//build/config/linux:x11" ]
 
     if (!is_chromeos) {
       sources += [
@@ -469,30 +472,44 @@ jumbo_component("base") {
     }
   }
 
-  if (use_x11 && use_aura) {
+  if ((use_x11 || ozone_platform_x11) && use_aura) {
     sources += [
-      "cursor/cursor_loader_x11.cc",
-      "cursor/cursor_loader_x11.h",
-      "cursor/cursor_x11.cc",
-      "x/selection_owner.cc",
-      "x/selection_owner.h",
-      "x/selection_requestor.cc",
-      "x/selection_requestor.h",
       "x/selection_utils.cc",
       "x/selection_utils.h",
-    ]
-    deps += [
-      "//ui/events/platform/x11",
-      "//ui/gfx/x",
+      "x/selection_owner.cc",
+      "x/selection_owner.h",
     ]
 
     if (!is_chromeos) {
       # These Aura X11 files aren't used on ChromeOS.
       sources += [
-        "dragdrop/os_exchange_data_provider_aurax11.cc",
-        "dragdrop/os_exchange_data_provider_aurax11.h",
+        "dragdrop/os_exchange_data_provider_aurax11_base.cc",
+        "dragdrop/os_exchange_data_provider_aurax11_base.h",
       ]
     }
+
+    if (use_x11 && use_aura) {
+      sources += [
+        "cursor/cursor_loader_x11.cc",
+        "cursor/cursor_loader_x11.h",
+        "cursor/cursor_x11.cc",
+        "x/selection_requestor.cc",
+        "x/selection_requestor.h",
+      ]
+
+      if (!is_chromeos) {
+        # These Aura X11 files aren't used on ChromeOS.
+        sources += [
+          "dragdrop/os_exchange_data_provider_aurax11.cc",
+          "dragdrop/os_exchange_data_provider_aurax11.h",
+        ]
+      }
+    }
+
+    deps += [
+      "//ui/events/platform/x11",
+      "//ui/gfx/x",
+    ]
   }
 
   if (use_aura) {