PaymentHandler: Fix a typo (camelCase to hacker_case).

This CL is initiated from:
  https://codereview.chromium.org/2844463002/diff/1/components/payments/mojom/payment_app.mojom#newcode51

BUG=661608

Review-Url: https://codereview.chromium.org/2839813005
Cr-Commit-Position: refs/heads/master@{#467357}
5 files changed