Revert "Stop polling in PDFExtensionClipboardTest."

This reverts commit 5e803e36.

Reason for revert: Collided with https://crrev.com/801651

Original change's description:
> Stop polling in PDFExtensionClipboardTest.
> 
> Now that https://crbug.com/755826 has been fixed, tests that read from
> the clipboard can use ClipboardMonitor to get notifications for when the
> clipboard changes, instead of polling the clipboard. This is more
> straight-forward, and hopefully more reliable.
> 
> Bug: 1121446
> Change-Id: I53291e0957ee2bad8a565bb0d80ef0628be40e2c
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2376765
> Reviewed-by: Stuart Langley <slangley@chromium.org>
> Commit-Queue: Lei Zhang <thestig@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#802025}

TBR=thestig@chromium.org,slangley@chromium.org

Change-Id: I20774cf96cf6a64c5f593852004a2c079e790320
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1121446
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2378742Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802037}

chrome/browser/pdf/pdf_extension_test.cc

@@ -97,8 +97,6 @@
 #include "ui/accessibility/ax_tree.h"
 #include "ui/accessibility/platform/ax_platform_node_delegate_base.h"
 #include "ui/base/clipboard/clipboard.h"
-#include "ui/base/clipboard/clipboard_monitor.h"
-#include "ui/base/clipboard/clipboard_observer.h"
 #include "ui/base/clipboard/test/test_clipboard.h"
 #include "ui/base/resource/resource_bundle.h"
 #include "ui/gfx/geometry/point.h"
@@ -1898,29 +1896,21 @@ IN_PROC_BROWSER_TEST_F(PDFExtensionInternalLinkClickTest, ShiftLeft) {
   EXPECT_EQ("page=2&zoom=100,0,200", url.ref());
 }
 
-class PDFExtensionClipboardTest : public PDFExtensionTest,
-                                  public ui::ClipboardObserver {
+class PDFExtensionClipboardTest : public PDFExtensionTest {
  public:
   PDFExtensionClipboardTest() : guest_contents_(nullptr) {}
   ~PDFExtensionClipboardTest() override {}
 
-  // PDFExtensionTest:
   void SetUpOnMainThread() override {
     PDFExtensionTest::SetUpOnMainThread();
     ui::TestClipboard::CreateForCurrentThread();
   }
+
   void TearDownOnMainThread() override {
     ui::Clipboard::DestroyClipboardForCurrentThread();
     PDFExtensionTest::TearDownOnMainThread();
   }
 
-  // ui::ClipboardObserver:
-  void OnClipboardDataChanged() override {
-    DCHECK(!clipboard_changed_);
-    clipboard_changed_ = true;
-    std::move(clipboard_quit_closure_).Run();
-  }
-
   void LoadTestComboBoxPdfGetGuestContents() {
     GURL test_pdf_url(embedded_test_server()->GetURL("/pdf/combobox_form.pdf"));
     guest_contents_ = LoadPdfGetGuestContents(test_pdf_url);
@@ -2017,30 +2007,45 @@ class PDFExtensionClipboardTest : public PDFExtensionTest,
   }
 
  private:
+  // Waits and polls the clipboard of a given |clipboard_buffer| until its
+  // contents reaches the length of |expected|. Then checks and see if the
+  // clipboard contents matches |expected|.
+  // TODO(thestig): Change this to avoid polling after https://crbug.com/755826
+  // has been fixed.
   void CheckClipboard(ui::ClipboardBuffer clipboard_buffer,
                       const std::string& expected) {
-    ui::ClipboardMonitor::GetInstance()->AddObserver(this);
-    DCHECK(!clipboard_changed_);
-    DCHECK(!clipboard_quit_closure_);
-
-    base::RunLoop run_loop;
-    clipboard_quit_closure_ = run_loop.QuitClosure();
-    run_loop.Run();
-
-    EXPECT_TRUE(clipboard_changed_);
-    clipboard_changed_ = false;
-    ui::ClipboardMonitor::GetInstance()->RemoveObserver(this);
-
     auto* clipboard = ui::Clipboard::GetForCurrentThread();
     std::string clipboard_data;
-    clipboard->ReadAsciiText(clipboard_buffer, /* data_dst=*/nullptr,
-                             &clipboard_data);
+    const std::string& last_data = last_clipboard_data_[clipboard_buffer];
+    if (last_data.size() == expected.size()) {
+      DCHECK_EQ(last_data, expected);
+      clipboard->ReadAsciiText(clipboard_buffer, /* data_dst = */ nullptr,
+                               &clipboard_data);
+      EXPECT_EQ(expected, clipboard_data);
+      return;
+    }
+
+    const bool expect_increase = last_data.size() < expected.size();
+    while (true) {
+      clipboard->ReadAsciiText(clipboard_buffer, /* data_dst = */ nullptr,
+                               &clipboard_data);
+      if (expect_increase) {
+        if (clipboard_data.size() >= expected.size())
+          break;
+      } else {
+        if (clipboard_data.size() <= expected.size())
+          break;
+      }
+
+      content::RunAllPendingInMessageLoop();
+    }
     EXPECT_EQ(expected, clipboard_data);
+
+    last_clipboard_data_[clipboard_buffer] = clipboard_data;
   }
 
-  base::RepeatingClosure clipboard_quit_closure_;
+  std::map<ui::ClipboardBuffer, std::string> last_clipboard_data_;
   WebContents* guest_contents_;
-  bool clipboard_changed_ = false;
 };
 
 IN_PROC_BROWSER_TEST_F(PDFExtensionClipboardTest,