Export Context::BackupIncumbentScope.

This patch adds a missing V8_EXPORT to Context::BackupIncumbentScope.

Bug: 
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: Ic4146ee1dad2f84222671fc54b4a463282356417
Reviewed-on: https://chromium-review.googlesource.com/762834Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49316}

include/v8.h

@@ -8852,7 +8852,7 @@ class V8_EXPORT Context {
    * stack.
    * https://html.spec.whatwg.org/multipage/webappapis.html#backup-incumbent-settings-object-stack
    */
-  class BackupIncumbentScope {
+  class V8_EXPORT BackupIncumbentScope {
    public:
     /**
      * |backup_incumbent_context| is pushed onto the backup incumbent settings