Specify correct request mode for SendBeacon

Bug: 695939
Change-Id: I88421009947673d4508a088f23f2b4e5a0926d67
Reviewed-on: https://chromium-review.googlesource.com/554423
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Takeshi Yoshino <tyoshino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#483384}
3 files changed