Fixes inconsistent conversion dates in logs

[tmancey]

Resolves brave/brave-browser#8689

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Verified that these changes build without errors on
  • Android
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that these changes pass automated tests (unit, browser, security tests) on
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that all lint errors/warnings are resolved (npm run lint)
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.
• Add appropriate QA labels (QA/Yes or QA/No) to include the closed issue in milestone
• Public documentation has been updated as necessary. For instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protection-Mode
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-compatibility-issues-with-tracking-protection
  • https://github.com/brave/brave-browser/wiki/QA-Guide

Test Plan:

• Ad conversions should trigger on average ~1 day unless passing the --brave-ads-debug command-line arg which will trigger on average ~5 minutes (and should reflect the epoch time in ad_conversions.json

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on.
• All relevant documentation has been updated.

[tmancey]

Linux failed, restarting Linux

[moritzhaller]

lgtm

[btlechowski]

LGTM

Verification passed on

Brave	1.8.14 Chromium: 80.0.3987.132 (Official Build) nightly (64-bit)
Revision	fcea73228632975e052eb90fcf6cd1752d3b42b4-refs/branch-heads/3987@{#974}}
OS	Ubuntu 18.04 LTS

Verified the min date for triggering the conversion is now respected

[2397:1:0316/175026.711449:INFO:ad_conversions.cc(179)] Started ad conversion timer for creative_instance_id bc6523ee-a790-487a-bb0e-7fda52b8af7e with creative set id 32439113-32d2-4b50-9497-405b7eedd1d5 which will trigger on 2020-03-17 23:20:51.000 UTC
[4738:1:0318/185811.441574:INFO:ad_conversions.cc(143)] Ad conversion for creative instance id bc6523ee-a790-487a-bb0e-7fda52b8af7e with creative set id 32439113-32d2-4b50-9497-405b7eedd1d5 triggered on 2020-03-17 23:20:51.000 UTC

Verified trigger date in ad_conversions.json is the same as in logs

Verified --brave-ads-debug shortens the process.

The log still does not show when conversion was triggered, it shows the min date that the conversion could trigger.