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Before the CL, ChromePaymentRequestFactory.sObserver was used before
assigned in production code.

To fix it, this CL null-check the observer, suffix it with
"ForTest", and annotate it with @Nullable.

This CL also add unit-tests for ChromePaymentRequestFactory which is
able to capture this crash.

Bug: 1143141
Binary-Size: Recovering after the wrong reduction of crrev.com/c/2499323
Change-Id: I6947fcee22d07c7641c9d22bb63defb1306c9079
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2504283Reviewed-by: Sahel Sharify <sahel@chromium.org>
Reviewed-by: Liquan (Max) Gu <maxlg@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Liquan (Max) Gu <maxlg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821990}