Replace several NSAssert with FML_CHECK/DCHECK to unblock the build (#…

…53048)

In flutter/buildroot#860, disabling `NSAssert` in release build was added, and #53005 tries to roll the change into the engine. However, the change is blocked due to several compilation errors in the engine.

This is due to the fact that Google's `DCHECK`(in flutter engine, `FML_DCHECK`) does not make unused variable warning even in release build, while traditional assertion macro like `assert` or `NSAssert` does. For example, the following code will be expanded like this:
code:
```c++
FML_DCHECK(status == kCVReturnSuccess && pxbuffer != nullptr) <<
   "Failed to create pixel buffer";
```
expanded code:
```c++
true || (status == kCVReturnSuccess && pxbuffer != nullptr)
    ? (void)0 ::
    : fml::LogMessageVoidify() &
          ::fml::LogMessage(::fml::kLogFatal, 0, 0, nullptr).stream()
```

However, equivalent code with `NSAssert` will make unused variable warning, since the expanded code doesn't have any reference to the variable:
code:
```c++
NSAssert(status == kCVReturnSuccess && pxbuffer != NULL, @"Failed to create pixel buffer.");
```

expanded code:
```c++
do {
} while (0)
```
To unblock the build, this CL replaces several `NSAssert` with `FML_DCHECK` in the engine codebase. This CL isn't aimed to replace all `NSAssert` with `FML_DCHECK` since discussions in flutter/buildroot#860 are not finalized whether to replace all `NSAssert` with `FML_DCHECK` or not.

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

shell/platform/darwin/macos/framework/Source/FlutterEmbedderExternalTextureTest.mm

@@ -59,7 +59,7 @@ - (CVPixelBufferRef)pixelBuffer {
   CVPixelBufferRef pxbuffer = NULL;
   CVReturn status = CVPixelBufferCreate(kCFAllocatorDefault, _width, _width, _pixelFormatType,
                                         (__bridge CFDictionaryRef)options, &pxbuffer);
-  NSAssert(status == kCVReturnSuccess && pxbuffer != NULL, @"Failed to create pixel buffer.");
+  FML_CHECK(status == kCVReturnSuccess && pxbuffer != nullptr) << "Failed to create pixel buffer";
   return pxbuffer;
 }
 

shell/platform/darwin/macos/framework/Source/FlutterEmbedderKeyResponder.mm

@@ -167,9 +167,10 @@ static uint64_t GetLogicalKeyForEvent(NSEvent* event, uint64_t physicalKey) {
     uint32_t* keyLabel = DecodeUtf16(keyLabelUtf16, &keyLabelLength);
     if (keyLabelLength == 1) {
       uint32_t keyLabelChar = *keyLabel;
-      NSCAssert(!IsControlCharacter(keyLabelChar) && !IsUnprintableKey(keyLabelChar),
-                @"Unexpected control or unprintable keylabel 0x%x", keyLabelChar);
-      NSCAssert(keyLabelChar <= 0x10FFFF, @"Out of range keylabel 0x%x", keyLabelChar);
+      FML_DCHECK(!IsControlCharacter(keyLabelChar) && !IsUnprintableKey(keyLabelChar))
+          << "Unexpected control or unprintable keylabel 0x" << std::hex << keyLabelChar;
+      FML_DCHECK(keyLabelChar <= 0x10FFFF)
+          << "Out of range keylabel 0x" << std::hex << keyLabelChar;
       character = keyLabelChar;
     }
     delete[] keyLabel;