tweeting about a tip doesn't include youtube or twitch in canned tweet

[LaurenWags]

Description

After tipping a YouTube channel or Twitch channel, you have the option to Tweet about your tip. When you do this, the canned tweet text doesn't include 'on YouTube' or 'on Twitch' as appropriate.

Steps to Reproduce

1. Navigate to verified YT or Twitch channel.
2. Tip a video, stream, etc.
3. On 'Thank you' banner, click on Tweet button.

Actual result:

On the tweet, you will not see 'on YouTube' or 'on Twitch' as appropriate. ex: https://twitter.com/bravelaurenwags/status/1131666887694311424

Expected result:

Should have 'on YouTube' or 'on Twitch' if appropriate.

Reproduces how often:

easily

Brave version (brave://version info)

Brave	0.67.55 Chromium: 75.0.3770.38 (Official Build) nightly(64-bit)
Revision	3860105745f2b12537da9e9f048f14c3f52ba970-refs/branch-heads/3770@{#618}
OS	Mac OS X

Version/Channel Information:

• Can you reproduce this issue with the current release? n/a
• Can you reproduce this issue with the beta channel? n/a
• Can you reproduce this issue with the dev channel? n/a
• Can you reproduce this issue with the nightly channel? yes

Other Additional Information:

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

Miscellaneous Information:

cc @brave/legacy_qa @mandar-brave @alexwykoff @jenn-rhim