[linux] Fix text selection via Shift+Home/End (flutter#24623)

Adds TextInputModel::SelectToBeginning/End() and applies when shift key is pressed.

shell/platform/common/text_input_model.cc

@@ -239,6 +239,24 @@ bool TextInputModel::MoveCursorToEnd() {
   return true;
 }
 
+bool TextInputModel::SelectToBeginning() {
+  size_t min_pos = editable_range().start();
+  if (selection_.collapsed() && selection_.position() == min_pos) {
+    return false;
+  }
+  selection_ = TextRange(selection_.base(), min_pos);
+  return true;
+}
+
+bool TextInputModel::SelectToEnd() {
+  size_t max_pos = editable_range().end();
+  if (selection_.collapsed() && selection_.position() == max_pos) {
+    return false;
+  }
+  selection_ = TextRange(selection_.base(), max_pos);
+  return true;
+}
+
 bool TextInputModel::MoveCursorForward() {
   // If there's a selection, move to the end of the selection.
   if (!selection_.collapsed()) {

shell/platform/common/text_input_model.h

@@ -155,6 +155,22 @@ class TextInputModel {
   // Returns true if the cursor could be moved.
   bool MoveCursorToEnd();
 
+  // Attempts to select text from the cursor position to the beginning.
+  //
+  // If composing is active, the selection is applied to the beginning of the
+  // composing range; otherwise, it is applied to the beginning of the text.
+  //
+  // Returns true if the selection could be applied.
+  bool SelectToBeginning();
+
+  // Attempts to select text from the cursor position to the end.
+  //
+  // If composing is active, the selection is applied to the end of the
+  // composing range; otherwise, it is moved to the end of the text.
+  //
+  // Returns true if the selection could be applied.
+  bool SelectToEnd();
+
   // Gets the current text as UTF-8.
   std::string GetText() const;