SlabAllocator: Remove note about C++17

We can't use aligned_alloc because it is only available starting from
macOS 10.15 and above. Now that we have C++17 the only thing blocking
use from using aligned_alloc is the macOS version, so note that.

Bug: dawn:824

Change-Id: Icb3bc22047193c189932dea4d10073dabb0cf32a
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/75065
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/common/SlabAllocator.cpp

@@ -228,7 +228,8 @@ void SlabAllocatorImpl::GetNewSlab() {
         return;
     }
 
-    // TODO(crbug.com/dawn/824): Use aligned_alloc with C++17.
+    // TODO(crbug.com/dawn/824): Use aligned_alloc when possible. It should be available with
+    // C++17 but on macOS it also requires macOS 10.15 to work.
     char* allocation = new char[mTotalAllocationSize];
     char* alignedPtr = AlignPtr(allocation, mAllocationAlignment);