Support configuration of preferences during runtime (issue #1709)

- Preferences are now associated with a CefRequestContext instead of
  being stored globally.
- Add methods to CefRequestContext for getting/setting preferences.

libcef/common/values_impl.h

@@ -49,6 +49,10 @@ class CefValueImpl : public CefValue {
                                    bool new_read_only,
                                    CefValueController* new_controller);
 
+  // Returns a reference to the underlying data. Access must be protected by
+  // calling AcquireLock/ReleaseLock.
+  base::Value* GetValueUnsafe() const;
+
   // CefValue methods.
   bool IsValid() override;
   bool IsOwned() override;
@@ -104,10 +108,6 @@ class CefValueImpl : public CefValue {
   void AcquireLock();
   void ReleaseLock();
 
-  // Returns a reference to the underlying data. Access must be protected by
-  // calling AcquireLock/ReleaseLock.
-  base::Value* GetValueUnsafe() const;
-
   // Access to all members must be protected by |lock_|.
   base::Lock lock_;