GWP-ASan: Increase maximum number of metadata

Due to performance regressions I've reduced the percentage of processes
GWP-ASan is enabled for. As a result, I've been able to proportionally
increase the amount of memory we use per process. As a result, the
previous maximum metadata of 255 no longer sufficies. Increase
MetadataIdx to be a 16-bit type and increase the maximum metadata to
something more reasonable.

Change-Id: If031dec3732ccf3ba69bdafdca2b03cb274bd9ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1761477
Commit-Queue: Vlad Tsyrklevich <vtsyrklevich@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>
Auto-Submit: Vlad Tsyrklevich <vtsyrklevich@chromium.org>
Reviewed-by: Vitaly Buka <vitalybuka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688424}

components/gwp_asan/common/allocator_state.h

@@ -39,14 +39,14 @@ class GuardedPageAllocator;
 
 class AllocatorState {
  public:
-  using MetadataIdx = uint8_t;
+  using MetadataIdx = uint16_t;
   using SlotIdx = uint16_t;
 
   // Maximum number of virtual memory slots (guard-page buffered pages) this
   // class can allocate.
   static constexpr size_t kMaxSlots = 4096;
   // Maximum number of concurrent allocations/metadata this class can allocate.
-  static constexpr size_t kMaxMetadata = 255;
+  static constexpr size_t kMaxMetadata = 2048;
   // Invalid metadata index.
   static constexpr MetadataIdx kInvalidMetadataIdx = kMaxMetadata;