Specify an internal ID to ::-webkit-file-upload-button

This CL makes the code robust for UA shadow tree changes.
This CL has no behavior changes.

Bug: 1040828
Change-Id: I373b5c01255149890f25b194f3777199dbaad2a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2011465Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733518}

third_party/blink/renderer/core/html/forms/file_input_type.cc

@@ -33,6 +33,7 @@
 #include "third_party/blink/renderer/core/html/forms/form_controller.h"
 #include "third_party/blink/renderer/core/html/forms/form_data.h"
 #include "third_party/blink/renderer/core/html/forms/html_input_element.h"
+#include "third_party/blink/renderer/core/html/shadow/shadow_element_names.h"
 #include "third_party/blink/renderer/core/html_names.h"
 #include "third_party/blink/renderer/core/input_type_names.h"
 #include "third_party/blink/renderer/core/inspector/console_message.h"
@@ -324,14 +325,17 @@ void FileInputType::CreateShadowSubtree() {
           GetElement().Multiple() ? IDS_FORM_MULTIPLE_FILES_BUTTON_LABEL
                                   : IDS_FORM_FILE_BUTTON_LABEL)));
   button->SetShadowPseudoId(AtomicString("-webkit-file-upload-button"));
+  button->setAttribute(html_names::kIdAttr,
+                       shadow_element_names::FileUploadButton());
   button->SetActive(GetElement().CanReceiveDroppedFiles());
   GetElement().UserAgentShadowRoot()->AppendChild(button);
 }
 
 HTMLInputElement* FileInputType::UploadButton() const {
-  Node* node = GetElement().UserAgentShadowRoot()->firstChild();
-  CHECK(!node || IsA<HTMLInputElement>(node));
-  return To<HTMLInputElement>(node);
+  Element* element = GetElement().UserAgentShadowRoot()->getElementById(
+      shadow_element_names::FileUploadButton());
+  CHECK(!element || IsA<HTMLInputElement>(element));
+  return To<HTMLInputElement>(element);
 }
 
 void FileInputType::DisabledAttributeChanged() {

third_party/blink/renderer/core/html/shadow/shadow_element_names.cc

@@ -104,6 +104,11 @@ const AtomicString& TextFieldContainer() {
   return name;
 }
 
+const AtomicString& FileUploadButton() {
+  DEFINE_STATIC_LOCAL(AtomicString, name, ("file-upload-button"));
+  return name;
+}
+
 const AtomicString& OptGroupLabel() {
   DEFINE_STATIC_LOCAL(AtomicString, name, ("optgroup-label"));
   return name;

third_party/blink/renderer/core/html/shadow/shadow_element_names.h

@@ -52,6 +52,7 @@ const AtomicString& PasswordRevealButton();
 CORE_EXPORT const AtomicString& SliderThumb();
 const AtomicString& SliderTrack();
 const AtomicString& TextFieldContainer();
+const AtomicString& FileUploadButton();
 const AtomicString& OptGroupLabel();
 
 }  // namespace shadow_element_names

third_party/blink/web_tests/fast/forms/file/input-file-label.html

@@ -10,33 +10,37 @@
 <p>This tests the label of a file chooser button.</p>
 <div id="console"></div>
 <script>
+function uploadButton(input) {
+  return internals.shadowRoot(input).getElementById('file-upload-button');
+}
+
 if (window.testRunner) {
     var file = document.getElementById('single_file');
-    var button = internals.shadowRoot(file).firstChild;
+    var button = uploadButton(file);
     var label = button.getAttribute('value');
     var result = 'The label of a single file chooser button is "' + label + '".';
     label == 'Choose File' ? testPassed(result) : testFailed(result);
 
     file = document.getElementById('multiple_files');
-    button = internals.shadowRoot(file).firstChild;
+    button = uploadButton(file);
     label = button.getAttribute('value');
     result = 'The label of a multiple file chooser button is "' + label + '".';
     label == 'Choose Files' ? testPassed(result) : testFailed(result);
 
     file = document.getElementById('single_or_multiple_file');
-    button = internals.shadowRoot(file).firstChild;
+    button = uploadButton(file);
     label = button.getAttribute('value');
     result = 'Initially, the label of a file chooser button is "' + label + '".';
     label == 'Choose File' ? testPassed(result) : testFailed(result);
 
     file.setAttribute("multiple", "multiple");
-    button = internals.shadowRoot(file).firstChild;
+    button = uploadButton(file);
     label = button.getAttribute('value');
     result = 'Set "multiple" attribute, then the label of the file chooser button becomes "' + label + '".';
     label == 'Choose Files' ? testPassed(result) : testFailed(result);
 
     file.removeAttribute("multiple");
-    button = internals.shadowRoot(file).firstChild;
+    button = uploadButton(file);
     label = button.getAttribute('value');
     result = 'Unset "multiple" attribute, then the label of the file chooser button becomes "' + label + '".';
     label == 'Choose File' ? testPassed(result) : testFailed(result);