bluetooth: Add EXPORT macro to BluetoothAdapter::Observer.

Component build requires EXPORT macro when code from content
utilizes this class, as used in upcoming
https://codereview.chromium.org/1125133005

BUG=489330

Review URL: https://codereview.chromium.org/1140813004

Cr-Commit-Position: refs/heads/master@{#330626}

device/bluetooth/bluetooth_adapter.h

@@ -41,7 +41,7 @@ class DEVICE_BLUETOOTH_EXPORT BluetoothAdapter
     : public base::RefCounted<BluetoothAdapter> {
  public:
   // Interface for observing changes from bluetooth adapters.
-  class Observer {
+  class DEVICE_BLUETOOTH_EXPORT Observer {
    public:
     virtual ~Observer() {}