Add slot=selected-value support for <selectmenu>

We decided to support this in OpenUI:
https://github.com/openui/open-ui/issues/657#issuecomment-1414262186

This patch also adds a test for ::part(selected-value) which as far as I
can tell was not tested before.

Bug: 1121840
Change-Id: I90b18d12249872239bd41e1be460ed46176fd17b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4220472
Auto-Submit: Joey Arhar <jarhar@chromium.org>
Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Joey Arhar <jarhar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1102933}

content/test/data/accessibility/html/selectmenu-expected-blink.txt

@@ -4,9 +4,10 @@ rootWebArea focusable
 ++++++genericContainer
 ++++++++genericContainer ignored
 ++++++++++comboBoxMenuButton collapsed focusable value='Option 1' haspopup=listbox
-++++++++++++genericContainer
-++++++++++++++staticText name='Option 1'
-++++++++++++++++inlineTextBox name='Option 1'
+++++++++++++genericContainer ignored
+++++++++++++++genericContainer
+++++++++++++++++staticText name='Option 1'
+++++++++++++++++++inlineTextBox name='Option 1'
 ++++++++++++genericContainer ignored
 ++++++++++++++genericContainer ignored
 ++++++++genericContainer ignored

content/test/data/accessibility/html/selectmenu-expected-fuchsia.txt

@@ -4,9 +4,10 @@ UNKNOWN focusable has_input_focus
 ++++++UNKNOWN
 ++++++++UNKNOWN hidden
 ++++++++++UNKNOWN focusable actions='{DEFAULT}' value='Option 1'
-++++++++++++UNKNOWN actions='{DEFAULT}'
-++++++++++++++STATIC_TEXT label='Option 1' actions='{DEFAULT}'
-++++++++++++++++UNKNOWN label='Option 1'
+++++++++++++UNKNOWN hidden
+++++++++++++++UNKNOWN actions='{DEFAULT}'
+++++++++++++++++STATIC_TEXT label='Option 1' actions='{DEFAULT}'
+++++++++++++++++++UNKNOWN label='Option 1'
 ++++++++++++UNKNOWN hidden
 ++++++++++++++UNKNOWN hidden
 ++++++++UNKNOWN hidden