Send live feedback on drag state to source

Wayland apps expect to be updated during a drag operation regarding
what action, if any, the current target is willing to
accept. Implement this by adding a new method to
DragDropClientObserver.

[email protected]

Bug: 927324
Change-Id: Idecc2070cd3f7f9cfdbaa3b95aa718ff5e1398d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1735568
Commit-Queue: Fergus Dall <[email protected]>
Reviewed-by: Scott Violet <[email protected]>
Reviewed-by: Daichi Hirono <[email protected]>
Reviewed-by: Mitsuru Oshima (slow) <[email protected]>
Reviewed-by: Nic Hollingum <[email protected]>
Cr-Commit-Position: refs/heads/master@{#688865}
diff --git a/components/exo/data_source_unittest.cc b/components/exo/data_source_unittest.cc
index 22ca10b..35ccbe6 100644
--- a/components/exo/data_source_unittest.cc
+++ b/components/exo/data_source_unittest.cc
@@ -32,7 +32,7 @@
 
   // Overridden from DataSourceDelegate:
   void OnDataSourceDestroying(DataSource* source) override {}
-  void OnTarget(const std::string& mime_type) override {}
+  void OnTarget(const base::Optional<std::string>& mime_type) override {}
   void OnSend(const std::string& mime_type, base::ScopedFD fd) override {
     ASSERT_TRUE(
         base::WriteFileDescriptor(fd.get(), kTestData, strlen(kTestData)));