[Chromecast] Forward declare content::WebContents

Including the WebContents header fails when a target doesn't depend on
content. Instead, we should only forward declare these classes in the
Cast web primary interfaces (CastWebContents, CastWebView,
CastContentWindow).

Merge-With: eureka-internal/461423

Bug: None
Test: CQ
Change-Id: Id18c669c7d39f4ed847a8c4970f5d6c8847a234c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2454370Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Commit-Queue: Sean Topping <seantopping@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814466}

chromecast/browser/cast_web_view.h

@@ -16,9 +16,13 @@
 #include "chromecast/browser/cast_web_contents.h"
 #include "chromecast/ui/mojom/ui_service.mojom.h"
 #include "content/public/browser/bluetooth_chooser.h"
-#include "content/public/browser/web_contents.h"
 #include "url/gurl.h"
 
+namespace content {
+class RenderFrameHost;
+class WebContents;
+}  // namespace content
+
 namespace chromecast {
 
 class CastWebService;