Migrate TextInputPlugin API to TextRange (flutter#21854)

Replaces selection_base() and selection_extent() with selection() and
SetSelection(int, int) with SetSelection(range).

This also adds the following convenience methods to TextRange:
* reversed()
* Contains(size_t position)
* Contains(const TextRange& range)

as well as operator== for use in unit tests. When Flutter migrates to
C++20, we can replace that method with a default declaration.

shell/platform/common/cpp/text_input_model.cc

@@ -41,12 +41,11 @@ void TextInputModel::SetText(const std::string& text) {
   selection_ = TextRange(0);
 }
 
-bool TextInputModel::SetSelection(size_t base, size_t extent) {
-  size_t max_pos = text_.length();
-  if (base > max_pos || extent > max_pos) {
+bool TextInputModel::SetSelection(const TextRange& range) {
+  if (!text_range().Contains(range)) {
     return false;
   }
-  selection_ = TextRange(base, extent);
+  selection_ = range;
   return true;
 }
 

shell/platform/common/cpp/text_input_model.h

@@ -27,8 +27,8 @@ class TextInputModel {
 
   // Attempts to set the text selection.
   //
-  // Returns false if the base or extent are out of bounds.
-  bool SetSelection(size_t base, size_t extent);
+  // Returns false if the selection is not within the bounds of the text.
+  bool SetSelection(const TextRange& range);
 
   // Adds a Unicode code point.
   //
@@ -105,11 +105,8 @@ class TextInputModel {
   // GetText().
   int GetCursorOffset() const;
 
-  // The position where the selection starts.
-  int selection_base() const { return selection_.base(); }
-
-  // The position of the cursor.
-  int selection_extent() const { return selection_.extent(); }
+  // The current selection.
+  TextRange selection() const { return selection_; }
 
  private:
   // Deletes the current selection, if any.
@@ -118,6 +115,9 @@ class TextInputModel {
   // reset to the start of the selected range.
   bool DeleteSelected();
 
+  // Returns a range covering the entire text.
+  TextRange text_range() const { return TextRange(0, text_.length()); }
+
   std::u16string text_;
   TextRange selection_ = TextRange(0);
 };