[fuchsia] Add size check for content directory handle list

Add a check to make sure the content directory handle list size doesn't
exceed the 16 bit handle-id space.

Test: web_engine_unittests
Bug: 1073176
Change-Id: I0a90f5bcca35a3c0e29e605bda9c6439092d3d29
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2283556
Commit-Queue: Sharon Yang <yangsharon@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: David Dorwin <ddorwin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#786062}

base/process/launch_fuchsia.cc

@@ -103,6 +103,8 @@ fdio_spawn_action_t FdioSpawnActionSetName(const char* name) {
 uint32_t LaunchOptions::AddHandleToTransfer(
     HandlesToTransferVector* handles_to_transfer,
     zx_handle_t handle) {
+  CHECK_LE(handles_to_transfer->size(), std::numeric_limits<uint16_t>::max())
+      << "Number of handles to transfer exceeds total allowed";
   uint32_t handle_id = PA_HND(PA_USER1, handles_to_transfer->size());
   handles_to_transfer->push_back({handle_id, handle});
   return handle_id;