Cherrypick a PartitionAlloc fix.

This applies https://crrev.com/602042 to fix cookie tracking for large
no-op reallocs.

BUG=chromium:897585

Change-Id: I79114f26bee5a2691195649447197564419347f8
Reviewed-on: https://pdfium-review.googlesource.com/c/48911
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

third_party/base/allocator/partition_allocator/partition_alloc.cc

diff --git a/third_party/base/allocator/partition_allocator/partition_alloc.cc b/third_party/base/allocator/partition_allocator/partition_alloc.cc
index d0d58df..87a51fd 100644
--- a/third_party/base/allocator/partition_allocator/partition_alloc.cc
+++ b/third_party/base/allocator/partition_allocator/partition_alloc.cc
@@ -209,12 +209,10 @@
 
   // bucket->slot_size is the current size of the allocation.
   size_t current_size = page->bucket->slot_size;
-  if (new_size == current_size)
-    return true;
-
   char* char_ptr = static_cast<char*>(internal::PartitionPage::ToPointer(page));
-
-  if (new_size < current_size) {
+  if (new_size == current_size) {
+    // No need to move any memory around, but update size and cookie below.
+  } else if (new_size < current_size) {
     size_t map_size =
         internal::PartitionDirectMapExtent::FromPage(page)->map_size;