SlabAllocator: Use array delete for slab

This was a regression during a recent refactor:
https://dawn-review.googlesource.com/c/dawn/+/169382/5/src/dawn/common/SlabAllocator.cpp

Bug: crbug.com/1519789
Change-Id: I18dc34660287cd433aa3826bd24be312cfd073d1
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/171120
Reviewed-by: Austin Eng <enga@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ken Buchanan <kenrb@google.com>
diff --git a/src/dawn/common/SlabAllocator.cpp b/src/dawn/common/SlabAllocator.cpp
index 7e92555..3f095d6 100644
--- a/src/dawn/common/SlabAllocator.cpp
+++ b/src/dawn/common/SlabAllocator.cpp
@@ -62,7 +62,7 @@
         DAWN_ASSERT(slab->blocksInUse == 0);
         char* allocation = slab->allocation;
         slab->~Slab();  // Placement delete.
-        delete allocation;
+        delete[] allocation;
     }
 }