block drag and drop data from Brave inside webviews

[diracdeltas]

short-term fix for #10573

Test Plan:
not public yet but see https://bravesoftware.slack.com/archives/G2KN13Z8C/p1503104809000056

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.

Test Plan:

Reviewer Checklist:

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 #10589 into master will increase coverage by 0.02%.
The diff coverage is 86.66%.

@@            Coverage Diff             @@
##           master   #10589      +/-   ##
==========================================
+ Coverage   54.19%   54.22%   +0.02%     
==========================================
  Files         244      244              
  Lines       21109    21122      +13     
  Branches     3258     3260       +2     
==========================================
+ Hits        11440    11453      +13     
  Misses       9669     9669

Flag	Coverage Δ
#unittest	54.22% <86.66%> (+0.02%)	⬆️

Impacted Files	Coverage Δ
js/dndData.js	60% <86.66%> (+19.25%)	⬆️

[NejcZdovc]

went through posted steps in security channel and it's working correctly. I also tried DND on wetransfter page and it's still working correctly.