Add GN arg to disable Alloy bootstrap (see #3681, see #3685)

Set enable_alloy_bootstrap=false to build with Alloy bootstrap code
removed. Extension API is documented as deprecated in comments but
not compiled out with this arg.

include/capi/cef_extension_capi.h

@@ -33,7 +33,7 @@
 // by hand. See the translator.README.txt file in the tools directory for
 // more information.
 //
-// $hash=0ca2b788f70f8c9f5b2706d691d8e063be00ed19$
+// $hash=634054ad25154c30fb4ec630fe7fb79b0cf1f9b3$
 //
 
 #ifndef CEF_INCLUDE_CAPI_CEF_EXTENSION_CAPI_H_
@@ -54,6 +54,8 @@ struct _cef_request_context_t;
 /// Object representing an extension. Methods may be called on any thread unless
 /// otherwise indicated.
 ///
+/// WARNING: This API is deprecated and will be removed in ~M127.
+///
 typedef struct _cef_extension_t {
   ///
   /// Base structure.