Cosmetic scriptlets not injected in iframes

[antonok-edm]

Currently, scriptlet injections from 3rd party contexts (i.e., cosmetic rules of the form ##+js()) are only injected into the top level frame of a website. This sometimes causes issues inside of 3rd-party iframes, such as youtube embedded players.

[bsclifton]

Labeled as P2 since this is causing ads to slip through for YouTube

[LaurenWags]

Verified passed with

Brave | 1.10.95 Chromium: 83.0.4103.106 (Official Build) (64-bit)
-- | --
Revision | ce7134bb3d95141cd18f1e65772a4247f282d950-refs/branch-heads/4103@{#694}
OS | macOS Version 10.14.6 (Build 18G3020)

• Verified test plan from Inject cosmetic filtering scriptlets in all frames brave-core#5878
• Reproduced the issue using 1.10.93 and test plan from PR. Using 1.10.93, encountered 4 videos with unskippable ads on https://www.pcmag.com/news/the-best-vr-games-for-2019. When using 1.10.95 and the same article (both clean and upgrade profiles), no unskippable ads were encountered.
• Visited various articles linked from https://www.pcmag.com/news/ which contained embedded YT videos. Confirmed no unskippable ads were shown on the embedded YT videos (in contrast, 1.10.93 did have a few unskippable ads on some articles).

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Verification PASSED on Mint 19.1 x64 using the following build:

Brave | 1.10.95 Chromium: 83.0.4103.106 (Official Build) (64-bit)
--- | ---
Revision | ce7134bb3d95141cd18f1e65772a4247f282d950-refs/branch-heads/4103@{#694}
OS | Linux

• Verified test plan from Inject cosmetic filtering scriptlets in all frames brave-core#5878
• Reproduced the issue using 1.10.93 and test plan from PR. Using 1.10.93, encountered 4 videos with unskippable ads on https://www.pcmag.com/news/the-best-vr-games-for-2019. When using 1.10.95 and the same article (both clean and upgrade profiles), no unskippable ads were encountered.
• Visited various articles linked from https://www.pcmag.com/news/ which contained embedded YT videos. Confirmed no unskippable ads were shown on the embedded YT videos (in contrast, 1.10.93 did have a few unskippable ads on some articles).

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Verified passed using

Brave	1.10.95 Chromium: 83.0.4103.106 (Official Build) (64-bit)
Revision	ce7134bb3d95141cd18f1e65772a4247f282d950-refs/branch-heads/4103@{#694}
OS	Windows 10 OS Version 1809 (Build 17763.914)

• Verified test plan from Inject cosmetic filtering scriptlets in all frames brave-core#5878
• Reproduced the issue using 1.10.93 and test plan from PR. Using 1.10.93, encountered 9 videos with ads (8 videos had unskippable ads, 1 video had a skippable ad) on https://www.pcmag.com/news/the-best-vr-games-for-2019. When using 1.10.95 and the same article (both clean and upgrade profiles), no unskippable ads were encountered.
• Visited various articles linked from https://www.pcmag.com/news/ which contained embedded YT videos. Confirmed no unskippable ads were shown on the embedded YT videos (in contrast, 1.10.93 did have a few unskippable ads on some articles).