IDL: Drop [Exposed] from UserAgent dictionary

Dictionary definition in Web IDL is actually mapping strings to objects.
So it doesn't make sense to have [Exposed] on it.

Currently the spec of UA defines it as an interface NavigatorUAData,
but this CL doesn't work for the change.


Tbr: yoavweiss@chromium.org
Bug: 839389
Change-Id: If41f713b1508ec0f19b242415eb78669872275bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1958271Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Commit-Queue: Hitoshi Yoshida <peria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#723663}

third_party/blink/renderer/core/frame/user_agent.idl

@@ -3,9 +3,7 @@
 // found in the LICENSE file.
 
 // https://github.com/WICG/ua-client-hints
-[
-    Exposed=Window
-] dictionary UserAgent {
+dictionary UserAgent {
   DOMString brand = "";
   DOMString version = "";
   DOMString platform = "";