Views UI: Improve middle-click pasting onto selected text behavior.

Previously, there was special-case code for middle-click pasting onto
selected text. This special-case code existed to try to emulate the
middle-click paste behavior of gedit.

However, gedit has some confusing behaviors. This CL modifies
middle-click pasting in the following ways:

 1. Middle-click pasting onto a selection now pastes the contents of
    the PRIMARY (selection) clipboard at the cursor position.

 2. The PRIMARY (selection) clipboard is no longer cleared after
    middle-clicking.

 3. This makes middle-click on UI surfaces like the Omnibox behave
    the same way as HTML text fields, which is a bonus.

BUG=364676

Review-Url: https://codereview.chromium.org/2826843002
Cr-Commit-Position: refs/heads/master@{#465775}

ui/views/controls/textfield/textfield_unittest.cc

@@ -2797,13 +2797,15 @@ TEST_F(TextfieldTest, SelectionClipboard) {
   EXPECT_EQ(gfx::Range(4, 4), textfield_->GetSelectedRange());
   EXPECT_TRUE(GetClipboardText(ui::CLIPBOARD_TYPE_SELECTION).empty());
 
-  // Middle clicking in the selection should clear the clipboard and selection.
+  // Middle clicking in the selection should insert the selection clipboard
+  // contents into the middle of the selection, and move the cursor to the end
+  // of the pasted content.
   SetClipboardText(ui::CLIPBOARD_TYPE_COPY_PASTE, "foo");
   textfield_->SelectRange(gfx::Range(2, 6));
   textfield_->OnMousePressed(middle);
-  EXPECT_STR_EQ("012301230123", textfield_->text());
-  EXPECT_EQ(gfx::Range(6, 6), textfield_->GetSelectedRange());
-  EXPECT_TRUE(GetClipboardText(ui::CLIPBOARD_TYPE_SELECTION).empty());
+  EXPECT_STR_EQ("0123foo01230123", textfield_->text());
+  EXPECT_EQ(gfx::Range(7, 7), textfield_->GetSelectedRange());
+  EXPECT_STR_EQ("foo", GetClipboardText(ui::CLIPBOARD_TYPE_SELECTION));
 
   // Double and triple clicking should update the clipboard contents.
   textfield_->SetText(ASCIIToUTF16("ab cd ef"));

ui/views/selection_controller.cc

@@ -80,20 +80,14 @@ bool SelectionController::OnMousePressed(
     }
   }
 
-  if (handles_selection_clipboard_ && event.IsOnlyMiddleMouseButton()) {
-    if (render_text->IsPointInSelection(event.location())) {
-      delegate_->OnBeforePointerAction();
-      render_text->ClearSelection();
-      delegate_->UpdateSelectionClipboard();
-      delegate_->OnAfterPointerAction(false, true);
-    } else if (!delegate_->IsReadOnly()) {
-      delegate_->OnBeforePointerAction();
-      const bool selection_changed =
-          render_text->MoveCursorTo(event.location(), false);
-      const bool text_changed = delegate_->PasteSelectionClipboard();
-      delegate_->OnAfterPointerAction(text_changed,
-                                      selection_changed | text_changed);
-    }
+  if (handles_selection_clipboard_ && event.IsOnlyMiddleMouseButton() &&
+      !delegate_->IsReadOnly()) {
+    delegate_->OnBeforePointerAction();
+    const bool selection_changed =
+        render_text->MoveCursorTo(event.location(), false);
+    const bool text_changed = delegate_->PasteSelectionClipboard();
+    delegate_->OnAfterPointerAction(text_changed,
+                                    selection_changed | text_changed);
   }
 
   return true;