[wasm] Reduce memory reservation limit

The n5x bot fails to *allocate* more memory just before we reach the
soft limit of 512 GiB for *reserved* memory. Thus reduce it a bit. This
still leaves enough room for ~38 wasm memories with full guard regions.

R=ahaas@chromium.org

Bug: v8:8196
Change-Id: I6049e2baa3fc8b99c9269f8cb7c013a6e77ebf9b
Reviewed-on: https://chromium-review.googlesource.com/1235924Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56084}

src/wasm/wasm-memory.cc

@@ -137,9 +137,7 @@ void* TryAllocateBackingStore(WasmMemoryTracker* memory_tracker, Heap* heap,
 constexpr size_t kAddressSpaceSoftLimit = 0x2100000000L;  // 132 GiB
 constexpr size_t kAddressSpaceHardLimit = 0x4000000000L;  // 256 GiB
 #elif V8_TARGET_ARCH_64_BIT
-// We set the limit to 1 TiB + 4 GiB so that there is room for mini-guards
-// once we fill everything up with full-sized guard regions.
-constexpr size_t kAddressSpaceSoftLimit = 0x8000000000L;   // 512 GiB
+constexpr size_t kAddressSpaceSoftLimit = 0x6000000000L;   // 384 GiB
 constexpr size_t kAddressSpaceHardLimit = 0x10100000000L;  // 1 TiB + 4 GiB
 #else
 constexpr size_t kAddressSpaceSoftLimit = 0x90000000;  // 2 GiB + 256 MiB