Cosmetic filter is not always working

[bsclifton]

Test plan

See brave/brave-core#2486

Description

For most sites, you can:

1. right click somewhere on site
2. choose Brave menu
3. choose Block element via selector
4. alert box with selector should show up

Step 4 (the alert) is not always happening. Even if you wait it out. This may be related to the changes made with brave/brave-core#2043

Steps to Reproduce

1. Visit https://www.reddit.com/ or https://www.benzworld.org/forums/w126-s-se-sec-sel-sd/ or another site with the problem
2. Right click on the ad, pick Block element via selector (as outlined above)

Actual result:

Nothing happens

Expected result:

Should be an input box with selector:

Reproduces how often:

100%

Brave version (brave://version info)

Version/Channel Information:

• Can you reproduce this issue with the current release? haven't tried
• Can you reproduce this issue with the beta channel? haven't tried
• Can you reproduce this issue with the dev channel? y
• Can you reproduce this issue with the nightly channel? y

Other Additional Information:

• Does the issue resolve itself when disabling Brave Shields? n
• Does the issue resolve itself when disabling Brave Rewards? n
• Is the issue reproducible on the latest version of Chrome? n

[btlechowski]

Verification passed on

Brave	0.68.113 Chromium: 76.0.3809.62 (Official Build) beta (64-bit)
Revision	7b77856b3aa34d72f246d12340fc1ded8b2c0e83-refs/branch-heads/3809@{#798}
OS	Ubuntu 18.04 LTS

Verified test plan from brave/brave-core#2486
Verified pages from the description.

Verification passed on

Brave	0.68.114 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 test plan from context menu fix for cosmetic filter brave-core#2486
• Verified pages from the description.

Verified passed with

Brave	0.68.116 Chromium: 76.0.3809.87 (Official Build) beta (64-bit)
Revision	111fe1e15d5ced26080a7dc239bcfe70f6c49aad-refs/branch-heads/3809@{#967}
OS	Mac OS X

• Verified test plan from context menu fix for cosmetic filter brave-core#2486