[ozone/wayland] Add a helper function to WaylandDataDevice to read the data from FD

It moves the code to read the data from FD to 'ReadDataFromFD' in
WaylandDataDevice to reuse it for drag-and-drop, as WaylandDataDevice
is the class to handle not only copy-and-paste but drag-and-drop and
drag-and-drop needs to read the data from FD as well.

It doesn't bring behavioral changes.

Bug: 578890, 875164
Change-Id: I460ab9df2b4d32bbdb0e71c753cb19c572df12ab
Reviewed-on: https://chromium-review.googlesource.com/1179505Reviewed-by: Maksim Sisov <msisov@igalia.com>
Reviewed-by: Antonio Gomes <tonikitoo@igalia.com>
Commit-Queue: Julie Jeongeun Kim <jkim@igalia.com>
Cr-Commit-Position: refs/heads/master@{#584047}

ui/ozone/platform/wayland/wayland_data_device.cc

@@ -41,20 +41,30 @@ void WaylandDataDevice::RequestSelectionData(const std::string& mime_type) {
   read_from_fd_closure_ =
       base::BindOnce(&WaylandDataDevice::ReadClipboardDataFromFD,
                      base::Unretained(this), std::move(fd), mime_type);
-  sync_callback_.reset(wl_display_sync(connection_->display()));
-  wl_callback_add_listener(sync_callback_.get(), &callback_listener_, this);
-  wl_display_flush(connection_->display());
+  RegisterSyncCallback();
 }
 
 void WaylandDataDevice::ReadClipboardDataFromFD(base::ScopedFD fd,
                                                 const std::string& mime_type) {
   std::string contents;
+  ReadDataFromFD(std::move(fd), &contents);
+  connection_->SetClipboardData(contents, mime_type);
+}
+
+void WaylandDataDevice::RegisterSyncCallback() {
+  DCHECK(!sync_callback_);
+  sync_callback_.reset(wl_display_sync(connection_->display()));
+  wl_callback_add_listener(sync_callback_.get(), &callback_listener_, this);
+  wl_display_flush(connection_->display());
+}
+
+void WaylandDataDevice::ReadDataFromFD(base::ScopedFD fd,
+                                       std::string* contents) {
+  DCHECK(contents);
   char buffer[1 << 10];  // 1 kB in bytes.
   ssize_t length;
   while ((length = read(fd.get(), buffer, sizeof(buffer))) > 0)
-    contents.append(buffer, length);
-
-  connection_->SetClipboardData(contents, mime_type);
+    contents->append(buffer, length);
 }
 
 std::vector<std::string> WaylandDataDevice::GetAvailableMimeTypes() {

ui/ozone/platform/wayland/wayland_data_device.h

@@ -37,6 +37,12 @@ class WaylandDataDevice {
  private:
   void ReadClipboardDataFromFD(base::ScopedFD fd, const std::string& mime_type);
 
+  // Registers display sync callback. Once it's called, it's reset.
+  void RegisterSyncCallback();
+
+  // Helper function to read data from fd.
+  void ReadDataFromFD(base::ScopedFD fd, std::string* contents);
+
   // wl_data_device_listener callbacks
   static void OnDataOffer(void* data,
                           wl_data_device* data_device,