Using RenderFrameHost::CopyImageAt for image copy

[darkdh]

fix #7388

Auditors: @bbondy, @bsclifton

Test Plan:

1. Go to https://www.brave.com/about/images/brave_icon_512x.png
2. Click "Copy Image" from context menu
3. The image should be copied to your clipboard instead of url
4. Paste it to somewhere else like slack channel to verify

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).
• 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. (Ask a Brave employee to help if you cannot access this document.)

Test Plan:

Reviewer Checklist:

• Request a security/privacy review as needed if one was not already requested.

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[codecov-io]

Codecov Report

Merging #13108 into master will increase coverage by 0.02%.
The diff coverage is 30.76%.

@@            Coverage Diff             @@
##           master   #13108      +/-   ##
==========================================
+ Coverage   56.31%   56.33%   +0.02%     
==========================================
  Files         280      278       -2     
  Lines       27591    27561      -30     
  Branches     4499     4498       -1     
==========================================
- Hits        15537    15526      -11     
+ Misses      12054    12035      -19

Flag	Coverage Δ
#unittest	56.33% <30.76%> (+0.02%)	⬆️

Impacted Files	Coverage Δ
js/constants/appConstants.js	100% <ø> (ø)	⬆️
js/stores/appStore.js	16.13% <ø> (+0.02%)	⬆️
app/browser/reducers/tabsReducer.js	41.9% <0%> (-0.62%)	⬇️
js/actions/appActions.js	19.78% <0%> (ø)	⬆️
js/contextMenus.js	18.53% <0%> (+0.08%)	⬆️
app/browser/tabs.js	24.21% <66.66%> (-0.15%)	⬇️
js/lib/imageUtil.js

[petemill]

This works great, I verified that an image is copied at it's 'natural' size rather than its rendered size, which is perfect 👌 . Tested by copying the image from https://codepen.io/anon/pen/bLRRWR

[petemill]

Just a note that when cherry-picking to 0.20.x, even though there will not be a merge error, we will need to change webContentsCache.getWebContents(tabId) to getWebContents(tabId).

[petemill]

0.22.x 7da7fcb
0.21.x 7b0b075
0.20.x ef0a205