discardable_memory: Use smaller chunks on Windows 32 bit.

Large chunks are suspected to be the cause for the linked bug (address space
fragmentation). Speculatively use a smaller chunk size to confirm the theory.

Only use a smaller size on Windows 32, as this increases the number of file
descriptors on all platforms, and the underlying issue (address space
fragmentation) is almost impossible on 64 bit architectures, and very unlikely
on Android.

Bug: 983348
Change-Id: Ic3ecc0a37f712be3fa6d45dfb01326b651f3f470
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1915379
Reviewed-by: Peng Huang <penghuang@chromium.org>
Commit-Queue: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715159}

components/discardable_memory/client/client_discardable_shared_memory_manager.cc

@@ -24,13 +24,21 @@
 #include "base/threading/thread_task_runner_handle.h"
 #include "base/trace_event/memory_dump_manager.h"
 #include "base/trace_event/trace_event.h"
+#include "build/build_config.h"
 #include "components/crash/core/common/crash_key.h"
 
 namespace discardable_memory {
 namespace {
 
 // Default allocation size.
+#if defined(OS_WIN) && defined(ARCH_CPU_32_BITS)
+// On Windows 32 bit, use a smaller chunk, as address space fragmentation may
+// make a 4MiB allocation impossible to fulfill in the browser process.
+// See crbug.com/983348 for details.
+const size_t kAllocationSize = 1 * 1024 * 1024;
+#else
 const size_t kAllocationSize = 4 * 1024 * 1024;
+#endif
 
 // Global atomic to generate unique discardable shared memory IDs.
 base::AtomicSequenceNumber g_next_discardable_shared_memory_id;