[iOS] fix Flutter Metal Layer retain cycle.

[jonahwilliams]

The display link and/or notification observer was keeping the FlutterMetalLayer alive. Add an invalidate method and call it in the IOSSurface dtor.

How do we test this?

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact "@test-exemption-reviewer" in the #hackers channel in Chat (don't just cc them here, they won't see it! Use Discord!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[jonahwilliams]

alternatively we could say that the devicelab tests not immediately crashing demonstrates that this works :)

[jmagman]

How do we test this?

Check out:

engine/shell/platform/darwin/ios/framework/Source/VsyncWaiterIosTest.mm

Lines 130 to 144 in 74785a7

	- (void)testReleasesLinkOnInvalidation {
	__weak CADisplayLink* weakLink;
	@autoreleasepool {
	auto thread_task_runner = CreateNewThread("VsyncWaiterIosTest");
	VSyncClient* vsyncClient = [[VSyncClient alloc]
	initWithTaskRunner:thread_task_runner
	callback:[](std::unique_ptr<flutter::FrameTimingsRecorder> recorder) {}];
	weakLink = [vsyncClient getDisplayLink];
	XCTAssertNotNil(weakLink);
	[vsyncClient invalidate];
	}
	// VSyncClient has released the CADisplayLink.
	XCTAssertNil(weakLink);
	}

engine/shell/platform/darwin/ios/framework/Source/vsync_waiter_ios.mm

Lines 42 to 46 in 74785a7

	VsyncWaiterIOS::~VsyncWaiterIOS() {
	// This way, we will get no more callbacks from the display link that holds a weak (non-nilling)
	// reference to this C++ object.
	[client_ invalidate];
	}

engine/shell/platform/darwin/ios/framework/Source/vsync_waiter_ios.mm

Lines 137 to 140 in 74785a7

	- (void)invalidate {
	[_displayLink invalidate];
	_displayLink = nil; // Break retain cycle.
	}

[knopp]

Another way to fix this: #54119

[chinmaygarde]

I have a preference for @knopp s patch. It really brittle for IOSSurfaceMetalImpeller to know to check if the layers is of a specific type to call a method on it.

[jonahwilliams]

Let's do the other one