Revert "Move site_isolation_policy from content/common to content/browser."

This reverts commit 1aed2aaf6d2d1354ce7b30088b1ff3c645230fd7.

Reason for revert: Causing merge conflicts on a CL that needs to be merged.  (See https://crbug.com/786505.)  Can land again afterward.

Original change's description:
> Move site_isolation_policy from content/common to content/browser.
> 
> The move allows returning
> ContentBrowserClient::GetOriginsRequiringDedicatedProcess from
> SiteIsolationPolicy::GetIsolatedOrigins.
> 
> No expected behavior change.
> 
> Bug: 792162
> Change-Id: Idd79fbddf138c2043ff1811a2d3d58165bcb198b
> Reviewed-on: https://chromium-review.googlesource.com/794303
> Commit-Queue: Ɓukasz Anforowicz <[email protected]>
> Reviewed-by: Alex Moshchuk <[email protected]>
> Cr-Commit-Position: refs/heads/master@{#521893}

[email protected],[email protected]

Change-Id: I419b78c56adb2126eb93367e29bc569a004308f7
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 792162
Reviewed-on: https://chromium-review.googlesource.com/810012
Reviewed-by: Charlie Reis <[email protected]>
Commit-Queue: Charlie Reis <[email protected]>
Cr-Commit-Position: refs/heads/master@{#521907}
diff --git a/content/browser/web_contents/web_contents_impl.cc b/content/browser/web_contents/web_contents_impl.cc
index e9ce6b0..f22cf700 100644
--- a/content/browser/web_contents/web_contents_impl.cc
+++ b/content/browser/web_contents/web_contents_impl.cc
@@ -86,6 +86,7 @@
 #include "content/common/page_messages.h"
 #include "content/common/page_state_serialization.h"
 #include "content/common/render_message_filter.mojom.h"
+#include "content/common/site_isolation_policy.h"
 #include "content/common/view_messages.h"
 #include "content/public/browser/ax_event_notification_details.h"
 #include "content/public/browser/browser_context.h"