Mojo: Fix wrapping of invalid SharedMemoryHandles.

When wrapping an invalid SharedMemoryHandle, we should always
just return a null ScopedSharedBufferHandle. Currently we return
a non-null ScopedSharedBufferHandle which wraps an invalid platform
handle. This will always fail to serialize and is generally bad.

Also fixes DiscardableSharedMemoryManager mojom to allow a null
shared buffer handle in the response to
AllocateLockedDiscardableSharedMemory. The existing client implementation
already handles the null case, so no additional code changes are required

BUG=674406

Review-Url: https://codereview.chromium.org/2583583002
Cr-Commit-Position: refs/heads/master@{#438955}

components/discardable_memory/public/interfaces/discardable_shared_memory_manager.mojom

@@ -10,7 +10,7 @@ interface DiscardableSharedMemoryManager {
   // Allocate a locked discardable shared memory segment.
   AllocateLockedDiscardableSharedMemory(
       uint32 size,
-      int32 id) => (handle<shared_buffer> memory);
+      int32 id) => (handle<shared_buffer>? memory);
   // Notify manager that a memory segment has been deleted.
   DeletedDiscardableSharedMemory(int32 id);
 };

mojo/public/cpp/system/platform_handle.cc

@@ -66,6 +66,13 @@ ScopedSharedBufferHandle WrapSharedMemoryHandle(
     const base::SharedMemoryHandle& memory_handle,
     size_t size,
     bool read_only) {
+#if defined(OS_POSIX) && !(defined(OS_MACOSX) && !defined(OS_IOS))
+  if (memory_handle.fd == base::kInvalidPlatformFile)
+    return ScopedSharedBufferHandle();
+#else
+  if (!memory_handle.IsValid())
+    return ScopedSharedBufferHandle();
+#endif
   MojoPlatformHandle platform_handle;
   platform_handle.struct_size = sizeof(MojoPlatformHandle);
   platform_handle.type = kPlatformSharedBufferHandleType;
@@ -96,6 +103,8 @@ MojoResult UnwrapSharedMemoryHandle(ScopedSharedBufferHandle handle,
                                     base::SharedMemoryHandle* memory_handle,
                                     size_t* size,
                                     bool* read_only) {
+  if (!handle.is_valid())
+    return MOJO_RESULT_INVALID_ARGUMENT;
   MojoPlatformHandle platform_handle;
   platform_handle.struct_size = sizeof(MojoPlatformHandle);