Fixed crash when fetching Rewards Widget estimated earnings - 1.21.x

[tmancey]

Uplift of #8114
Resolves brave/brave-browser#14447

• You have checked CI and the builds, lint, and tests all pass or are not related to your PR.
• You have tested your change on Nightly.
• The PR milestones match the branch they are landing to.

After you merge:

• The associated issue milestone is set to the smallest version that the changes is landed on.

[kjozwiak]

Restarting pre-init due to the following failure:

[2021-03-05T14:56:24.976Z] Searching s3://**** for object:'brave-browser-build-pr/issues/14447-1.21.x/1/windows/build-status.txt'
[2021-03-05T14:56:25.002Z] Search complete
groovy.lang.MissingPropertyException: No such property: FAILED_STAGE for class: Script1

Restarting macOS as build failed as per https://ci.brave.com/job/pr-brave-browser-issues-14447-1.21.x-macos/1/execution/node/165/log/.

13:55:35  FAILED: obj/components/safe_browsing/content/common/interfaces/safe_browsing.mojom.o 

Restarting post-init due to the following failure:

[2021-03-05T14:55:34.990Z] Searching s3://**** for object:'brave-browser-build-pr/issues/14447-1.21.x/1/windows/build-status.txt'
[2021-03-05T14:55:35.096Z] Search complete
groovy.lang.MissingPropertyException: No such property: FAILED_STAGE for class: Script1

[kjozwiak]

Uplift into 1.21.x approved after deliberating with @brave/uplift-approvers. QA has verified the PR on Nightly as per #8114 (comment).