feat: replace BrowserView with WebContentsView (#40759)

* feat: replace BrowserView with WebContentsView

Co-authored-by: Jeremy Rose <jeremya@chromium.org>

* Update appveyor.yml

---------

Co-authored-by: trop[bot] <37223003+trop[bot]@users.noreply.github.com>
Co-authored-by: Jeremy Rose <jeremya@chromium.org>

buildflags/BUILD.gn

@@ -9,7 +9,6 @@ buildflag_header("buildflags") {
   header = "buildflags.h"
 
   flags = [
-    "ENABLE_VIEWS_API=$enable_views_api",
     "ENABLE_PDF_VIEWER=$enable_pdf_viewer",
     "ENABLE_ELECTRON_EXTENSIONS=$enable_electron_extensions",
     "ENABLE_BUILTIN_SPELLCHECKER=$enable_builtin_spellchecker",

buildflags/buildflags.gni

@@ -3,8 +3,6 @@
 # found in the LICENSE file.
 
 declare_args() {
-  enable_views_api = true
-
   enable_pdf_viewer = true
 
   # Provide a fake location provider for mocking