[Impeller] fixed units for memory measurement (#53687)

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

Author: gaaclarke

impeller/renderer/backend/metal/allocator_mtl.mm

@@ -99,7 +99,8 @@ static bool SupportsLossyTextureCompression(id<MTLDevice> device) {
 }
 
 size_t DebugAllocatorStats::GetAllocationSizeMB() {
-  size_t new_value = size_ / 1000000;
+  // RAM is measured in MiB, thus a divisor of 2^20 instead of 1,000,000.
+  size_t new_value = size_ / (1024 * 1024);
   return new_value;
 }
 

impeller/renderer/backend/metal/allocator_mtl_unittests.mm

@@ -36,7 +36,7 @@
     TextureDescriptor desc;
     desc.format = PixelFormat::kR8G8B8A8UNormInt;
     desc.storage_mode = StorageMode::kDeviceTransient;
-    desc.size = {1000, 1000};
+    desc.size = {1024, 1024};
     auto texture_1 = allocator->CreateTexture(desc);
 
     EXPECT_EQ(allocator->DebugGetHeapUsage(), 0u);

impeller/renderer/backend/vulkan/allocator_vk.cc

@@ -519,7 +519,7 @@ size_t AllocatorVK::DebugGetHeapUsage() const {
     total_usage += budget.usage;
   }
   // Convert bytes to MB.
-  total_usage *= 1e-6;
+  total_usage /= (1024 * 1024);
   return total_usage;
 }