Skia appears to use DiscardableSharedMemory on all platforms.

https://cs.chromium.org/chromium/src/skia/ext/SkDiscardableMemory_chrome.cc?type=cs&q=AllocateLockedDiscardableMemory&sq=package:chromium&l=40
"""
SkDiscardableMemory* SkDiscardableMemory::Create(size_t bytes) {
  return new SkDiscardableMemoryChrome(
      base::DiscardableMemoryAllocator::GetInstance()
          ->AllocateLockedDiscardableMemory(bytes));
}
"""

But DiscardableSharedMemory is only implemented on Android.
"""
// Returns SUCCESS on platforms which do not support discardable pages.
DiscardableSharedMemory::LockResult LockPages(const SharedMemory& memory,
                                              size_t offset,
                                              size_t length) {
#if defined(OS_ANDROID)
  SharedMemoryHandle handle = memory.handle();
  if (handle.IsValid()) {
    int pin_result = ashmem_pin_region(handle.GetHandle(), offset, length);
    if (pin_result == ASHMEM_WAS_PURGED)
      return DiscardableSharedMemory::PURGED;
    if (pin_result < 0)
      return DiscardableSharedMemory::FAILED;
  }
#endif
  return DiscardableSharedMemory::SUCCESS;
}
"""