media: Use NiceMock<MockCdm> in MockCdmFactory

The MockCdmFactory just creates the MockCdm and should not expect what
calls will be made on the MockCdm. Hence, replace StrictMock with
NiceMock. Test cases that expect calls on MockCdm should get the MockCdm
via MockCdmFactory::GetCreatedCdm() and explicitly specify expectations
using EXPECT_CALL.

This is also consistent with googlemock guidelines [1]:
"""
Our general recommendation is to use nice mocks (not yet the default)
most of the time, use naggy mocks (the current default) when developing
or debugging tests, and use strict mocks only as the last resort.
"""

[1] https://github.com/google/googletest/blob/master/googlemock/docs/cook_book.md#the-nice-the-strict-and-the-naggy-nicestrictnaggy

Bug: 1018854
Test: Update test helper classes. No functionality change.
Change-Id: I197f83b221296beccd05b82d2b6ae030381178e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1890970
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Reviewed-by: John Rummell <jrummell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710980}

media/base/mock_filters.cc

@@ -7,9 +7,9 @@
 #include "base/logging.h"
 
 using ::testing::_;
+using ::testing::NiceMock;
 using ::testing::Return;
 using ::testing::SaveArg;
-using ::testing::StrictMock;
 
 MATCHER(NotEmpty, "") {
   return !arg.empty();
@@ -244,12 +244,13 @@ void MockCdmFactory::Create(
     before_creation_cb_.Run();
 
   // Create and return a new MockCdm. Keep a pointer to the created CDM so
-  // that tests can access it. Calls to GetCdmContext() can be ignored.
-  scoped_refptr<MockCdm> cdm = new StrictMock<MockCdm>(
+  // that tests can access it. Test cases that expect calls on MockCdm should
+  // get the MockCdm via MockCdmFactory::GetCreatedCdm() and explicitly specify
+  // expectations using EXPECT_CALL.
+  scoped_refptr<MockCdm> cdm = new NiceMock<MockCdm>(
       key_system, security_origin, session_message_cb, session_closed_cb,
       session_keys_change_cb, session_expiration_update_cb);
   created_cdm_ = cdm.get();
-  EXPECT_CALL(*created_cdm_.get(), GetCdmContext());
   cdm_created_cb.Run(std::move(cdm), "");
 }