ClipboardNonBacked: Remove unused versioning support.

Removed unused versioning support in ClipboardInternal. This allowed for
multiple ClipboardData items on a clipboard stack, but the stack size
was limited to 1, and no logic existed to exercise use of any stack
items except the top item on the stack. Therefore, this essentially
amounted to extra logic that wasn't actually needed or used.

Versioning support has been available since at least 2012, when
https://crrev.com/10829341 first introduced this file as a "sketch of
a clipboard implementation for linux_aura".

Change-Id: I7615a97bdd4ef038b0684c75536d2417d746bffe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2309549
Commit-Queue: Victor Costan <pwnall@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790406}

ui/base/clipboard/clipboard_non_backed.cc

@@ -33,14 +33,8 @@
 
 namespace ui {
 
-namespace {
-
-const size_t kMaxClipboardSize = 1;
-
-}  // namespace
-
 // Simple, internal implementation of a clipboard, handling things like format
-// conversion, versioning, etc.
+// conversion, sequence numbers, etc.
 class ClipboardInternal {
  public:
   ClipboardInternal() = default;
@@ -48,41 +42,35 @@ class ClipboardInternal {
   ~ClipboardInternal() = default;
 
   void Clear() {
-    sequence_number_++;
-    data_list_.clear();
+    ++sequence_number_;
+    data_.reset();
     ClipboardMonitor::GetInstance()->NotifyClipboardDataChanged();
   }
 
   uint64_t sequence_number() const { return sequence_number_; }
 
-  // Returns the data currently on the top of the clipboard stack, nullptr if
-  // the clipboard stack is empty.
-  const ClipboardData* GetData() const {
-    if (data_list_.empty())
-      return nullptr;
-    return data_list_.front().get();
-  }
+  // Returns the current clipboard data, which may be nullptr if nothing has
+  // been written since the last Clear().
+  const ClipboardData* GetData() const { return data_.get(); }
 
   // Returns true if the data on top of the clipboard stack has format |format|
   // or another format that can be converted to |format|.
   bool IsFormatAvailable(ClipboardInternalFormat format) const {
-    switch (format) {
-      case ClipboardInternalFormat::kText:
-        return HasFormat(ClipboardInternalFormat::kText) ||
-               HasFormat(ClipboardInternalFormat::kBookmark);
-      default:
-        return HasFormat(format);
+    if (format == ClipboardInternalFormat::kText) {
+      return HasFormat(ClipboardInternalFormat::kText) ||
+             HasFormat(ClipboardInternalFormat::kBookmark);
     }
+    return HasFormat(format);
   }
 
-  // Reads text from the data at the top of clipboard stack.
+  // Reads text from the ClipboardData.
   void ReadText(base::string16* result) const {
     std::string utf8_result;
     ReadAsciiText(&utf8_result);
     *result = base::UTF8ToUTF16(utf8_result);
   }
 
-  // Reads ASCII text from the data at the top of clipboard stack.
+  // Reads ASCII text from the ClipboardData.
   void ReadAsciiText(std::string* result) const {
     result->clear();
     const ClipboardData* data = GetData();
@@ -96,7 +84,7 @@ class ClipboardInternal {
       *result = data->bookmark_url();
   }
 
-  // Reads HTML from the data at the top of clipboard stack.
+  // Reads HTML from the ClipboardData.
   void ReadHTML(base::string16* markup,
                 std::string* src_url,
                 uint32_t* fragment_start,
@@ -119,7 +107,7 @@ class ClipboardInternal {
     *fragment_end = static_cast<uint32_t>(markup->length());
   }
 
-  // Reads RTF from the data at the top of clipboard stack.
+  // Reads RTF from the ClipboardData.
   void ReadRTF(std::string* result) const {
     result->clear();
     const ClipboardData* data = GetData();
@@ -129,7 +117,7 @@ class ClipboardInternal {
     *result = data->rtf_data();
   }
 
-  // Reads image from the data at the top of clipboard stack.
+  // Reads image from the ClipboardData.
   SkBitmap ReadImage() const {
     SkBitmap img;
     if (!HasFormat(ClipboardInternalFormat::kBitmap))
@@ -144,7 +132,7 @@ class ClipboardInternal {
     return img;
   }
 
-  // Reads data of type |type| from the data at the top of clipboard stack.
+  // Reads data of type |type| from the ClipboardData.
   void ReadCustomData(const base::string16& type,
                       base::string16* result) const {
     result->clear();
@@ -156,7 +144,7 @@ class ClipboardInternal {
                           data->custom_data_data().size(), type, result);
   }
 
-  // Reads bookmark from the data at the top of clipboard stack.
+  // Reads bookmark from the ClipboardData.
   void ReadBookmark(base::string16* title, std::string* url) const {
     if (title)
       title->clear();
@@ -182,10 +170,11 @@ class ClipboardInternal {
     *result = data->custom_data_data();
   }
 
-  // Writes |data| to the top of the clipboard stack.
+  // Writes |data| to the ClipboardData.
   void WriteData(std::unique_ptr<ClipboardData> data) {
     DCHECK(data);
-    AddToListEnsuringSize(std::move(data));
+    ++sequence_number_;
+    data_ = std::move(data);
     ClipboardMonitor::GetInstance()->NotifyClipboardDataChanged();
   }
 
@@ -201,26 +190,14 @@ class ClipboardInternal {
   }
 
  private:
-  // True if the data on top of the clipboard stack has format |format|.
+  // True if the ClipboardData has format |format|.
   bool HasFormat(ClipboardInternalFormat format) const {
     const ClipboardData* data = GetData();
     return data ? data->format() & static_cast<int>(format) : false;
   }
 
-  void AddToListEnsuringSize(std::unique_ptr<ClipboardData> data) {
-    DCHECK(data);
-    sequence_number_++;
-    data_list_.push_front(std::move(data));
-
-    // If the size of list becomes more than the maximum allowed, we delete the
-    // last element.
-    if (data_list_.size() > kMaxClipboardSize) {
-      data_list_.pop_back();
-    }
-  }
-
-  // Stack containing various versions of ClipboardData.
-  std::list<std::unique_ptr<ClipboardData>> data_list_;
+  // Current ClipboardData.
+  std::unique_ptr<ClipboardData> data_;
 
   // Sequence number uniquely identifying clipboard state.
   uint64_t sequence_number_ = 0;