Use a test clipboard to avoid OS clipboard contention.

Bug: 396477
Change-Id: Ie64abe2a732405d44aec185e3f49907a5fc799a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1618747Reviewed-by: Michael Wasserman <msw@chromium.org>
Commit-Queue: David Bienvenu <davidbienvenu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661375}

ui/views/BUILD.gn

@@ -1092,6 +1092,7 @@ test("views_unittests") {
     "//ui/base",
     "//ui/base:test_support",
     "//ui/base/clipboard",
+    "//ui/base/clipboard:clipboard_test_support",
     "//ui/base/ime/init",
     "//ui/compositor:test_support",
     "//ui/events:dom_keycode_converter",

ui/views/controls/textfield/textfield_unittest.cc

@@ -25,6 +25,7 @@
 #include "ui/aura/window.h"
 #include "ui/base/clipboard/clipboard.h"
 #include "ui/base/clipboard/scoped_clipboard_writer.h"
+#include "ui/base/clipboard/test/test_clipboard.h"
 #include "ui/base/dragdrop/drag_drop_types.h"
 #include "ui/base/emoji/emoji_panel_helper.h"
 #include "ui/base/ime/constants.h"
@@ -406,6 +407,14 @@ class TextfieldTest : public ViewsTestBase, public TextfieldController {
     ViewsTestBase::TearDown();
   }
 
+  void SetUp() override {
+    // OS clipboard is a global resource, which causes flakiness when unit tests
+    // run in parallel. So, use a per-instance test clipboard.
+    ui::Clipboard::SetClipboardForCurrentThread(
+        std::make_unique<ui::TestClipboard>());
+    ViewsTestBase::SetUp();
+  }
+
   ui::ClipboardType GetAndResetCopiedToClipboard() {
     ui::ClipboardType clipboard_type = copied_to_clipboard_;
     copied_to_clipboard_ = ui::CLIPBOARD_TYPE_LAST;
@@ -2268,7 +2277,6 @@ TEST_F(TextfieldTest, Yank) {
 
 TEST_F(TextfieldTest, CutCopyPaste) {
   InitTextfield();
-
   // Ensure IDS_APP_CUT cuts.
   textfield_->SetText(ASCIIToUTF16("123"));
   textfield_->SelectAll(false);