[Chromecast] Include definitions file for CastWebView

The CastWebView interface declares some non-virtual methods, so we need
to build the definitions file alongside the header.

Bug: None
Test: CQ
Change-Id: I62af73be8be736aca072bb29be755ffc68dd5b77
Reviewed-on: https://chromium-review.googlesource.com/1045249Reviewed-by: Sergey Volk <servolk@chromium.org>
Reviewed-by: Stephen Lanham <slan@chromium.org>
Commit-Queue: Sean Topping <seantopping@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556209}

chromecast/browser/BUILD.gn

@@ -47,7 +47,6 @@ cast_source_set("browser") {
     "cast_resource_dispatcher_host_delegate.h",
     "cast_web_contents_manager.cc",
     "cast_web_contents_manager.h",
-    "cast_web_view.cc",
     "cast_web_view_default.cc",
     "cast_web_view_default.h",
     "cast_web_view_factory.cc",
@@ -279,6 +278,7 @@ cast_source_set("browser") {
 cast_source_set("public") {
   sources = [
     "cast_content_window.h",
+    "cast_web_view.cc",
     "cast_web_view.h",
   ]