Replace scoped_ptr with unique_ptr everywhere

But keep #including scoped_ptr.h in .h files, so as not to break
WebRTC users who expect those .h files to give them rtc::scoped_ptr.

BUG=webrtc:5520

Review-Url: https://codereview.webrtc.org/1937693002
Cr-Commit-Position: refs/heads/master@{#12581}

webrtc/sdk/objc/Framework/UnitTests/RTCIceCandidateTest.mm

@@ -10,6 +10,8 @@
 
 #import <Foundation/Foundation.h>
 
+#include <memory>
+
 #include "webrtc/base/gunit.h"
 
 #import "NSString+StdString.h"
@@ -32,7 +34,7 @@
                                                       sdpMLineIndex:0
                                                              sdpMid:@"audio"];
 
-  rtc::scoped_ptr<webrtc::IceCandidateInterface> nativeCandidate =
+  std::unique_ptr<webrtc::IceCandidateInterface> nativeCandidate =
       candidate.nativeCandidate;
   EXPECT_EQ("audio", nativeCandidate->sdp_mid());
   EXPECT_EQ(0, nativeCandidate->sdp_mline_index());

webrtc/sdk/objc/Framework/UnitTests/RTCMediaConstraintsTest.mm

@@ -10,6 +10,8 @@
 
 #import <Foundation/Foundation.h>
 
+#include <memory>
+
 #include "webrtc/base/gunit.h"
 
 #import "NSString+StdString.h"
@@ -29,7 +31,7 @@
   RTCMediaConstraints *constraints = [[RTCMediaConstraints alloc]
       initWithMandatoryConstraints:mandatory
                optionalConstraints:optional];
-  rtc::scoped_ptr<webrtc::MediaConstraints> nativeConstraints =
+  std::unique_ptr<webrtc::MediaConstraints> nativeConstraints =
       [constraints nativeConstraints];
 
   webrtc::MediaConstraintsInterface::Constraints nativeMandatory =