Fix ads should only be sustained after the page has fully loaded

[tmancey]

Description

Fix Ads should only be sustained after the page has fully loaded

Steps to Reproduce

1. View an Ad

Actual result:

Ad is sustained 10 seconds after page starts to load

Expected result:

Ad should be sustained 10 seconds after page has fully loaded

Reproduces how often:

Brave version (brave://version info)

Version/Channel Information:

• Can you reproduce this issue with the current release? Yes
• Can you reproduce this issue with the beta channel? Yes
• Can you reproduce this issue with the dev channel? Yes
• Can you reproduce this issue with the nightly channel? Yes

Other Additional Information:

• Does the issue resolve itself when disabling Brave Shields? No
• Does the issue resolve itself when disabling Brave Rewards? No
• Is the issue reproducible on the latest version of Chrome? No

Miscellaneous Information:

[GeetaSarvadnya]

Verification passed on

Brave	0.68.116 Chromium: 76.0.3809.87 (Official Build) beta (64-bit)
Revision	111fe1e15d5ced26080a7dc239bcfe70f6c49aad-refs/branch-heads/3809@{#967}
OS	Windows 10 OS Version 1803 (Build 17134.523)

• Verified the test plan from Fix ads should only be sustained after the page has fully loaded brave-core#2658

Verification passed on

Brave	0.68.118 Chromium: 76.0.3809.87 (Official Build) beta (64-bit)
Revision	111fe1e15d5ced26080a7dc239bcfe70f6c49aad-refs/branch-heads/3809@{#967}
OS	Ubuntu 18.04 LTS

• Verified the test plan from Fix ads should only be sustained after the page has fully loaded brave-core#2658

Verified passed with

Brave	0.68.130 Chromium: 76.0.3809.100 (Official Build) (64-bit)
Revision	ed9d447d30203dc5069e540f05079e493fc1c132-refs/branch-heads/3809@{#990}
OS	Mac OS X

• Verified test plan from Fix ads should only be sustained after the page has fully loaded brave-core#2658 5x.
  
• Occasionally did encounter Brave Ads target URLs that redirect to a different hostname are not sustained #4026