[WebLayer] Add payments::RegisterProfilePrefs for WebLayer

WebLayerPaymentRequestFactory's prefsCanMakePayment()[1] depends on
the profile prefs being registered, so this CL adds the registration to
weblayer's browser_context_impl, similar to its chrome counterpart[2].

[1] https://source.chromium.org/chromium/chromium/src/+/master:weblayer/browser/java/org/chromium/weblayer_private/payments/WebLayerPaymentRequestFactory.java;l=60;drc=b3fd96a6c7479e8721ea7ea56c4340a472ca7e13
[2] https://source.chromium.org/chromium/chromium/src/+/master:chrome/browser/prefs/browser_prefs.cc;l=815;drc=29dc236a1c6f8f3cb12af26986f8484f7ff7a55b

Bug: 1025619
Change-Id: Ie31948cf6030b200b4c9c1dcbc1c48c6167f0a45
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2551319
Commit-Queue: Liquan (Max) Gu <maxlg@chromium.org>
Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#830307}