Boost thread priority while loading WebAuthn

The WebAuthn DLL seems to load DLL too.

This is a follow up on :
  https://chromium-review.googlesource.com/c/chromium/src/+/2376878

From these hang reports:
  87b0bdd85d6426a8
  ed2e7a916cbfee87
  b6ab544feec07866
  3c4e02aff6f7ad7a
  ce79755a1e1abf2f

Stackframe:
  LdrResolveDelayLoadedAPI
 _delayLoadHelper2
 _tailMerge_cryptngc_dll
 WebAuthNIsUserVerifyingPlatformAuthenticatorAvailable
 device::WinWebAuthnApiAuthenticator::IsUserVerifyingPlatformAuthenticatorAvailable

R=joenotcharles@google.com, martinkr@google.com

Bug: 973868
Change-Id: I0e6fde3ce94f2e1e8105127df4ba492970821203
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2386100Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Reviewed-by: Joe Mason <joenotcharles@chromium.org>
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803894}

device/fido/win/webauthn_api.cc

@@ -95,6 +95,9 @@ class WinWebAuthnApiImpl : public WinWebAuthnApi {
   HRESULT IsUserVerifyingPlatformAuthenticatorAvailable(
       BOOL* available) override {
     DCHECK(is_bound_);
+    // Mitigate the issues caused by loading DLLs on a background thread
+    // (http://crbug/973868).
+    SCOPED_MAY_LOAD_LIBRARY_AT_BACKGROUND_PRIORITY();
     return is_user_verifying_platform_authenticator_available_(available);
   }