View page source doesn't work for local files

[cndouglas]

Test plan

#8404 (comment)

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• Did you search for similar issues before submitting this one?
  Yes.

• Describe the issue you encountered:
  For local files (file://URL scheme), the View Page Source option does not work. The problem is most apparent with "rendered" files (e.g., HTML files).
  There are no errors or warnings in the browser-level dev tools console.

• Platform (Win7, 8, 10? macOS? Linux distro?):
  macOS 10.12.x

• Brave Version (revision SHA):
  0.14.1 (3de60d5)

• Steps to reproduce:

  1. Open a local HTML file with File > Open File….
  2. Right-click anywhere and select View Page Source.

• Actual result:
  Nothing happens.

• Expected result:
  The source should be opened in another tab.

• Will the steps above reproduce in a fresh profile? If not what other info can be added?
  Probably.

• Is this an issue in the currently released version?
  Yes, 0.14.1.

• Can this issue be consistently reproduced?
  Yes.

• Extra QA steps:

  1. Open a local HTML file with File > Open File….
  2. Right-click anywhere and select View Page Source.
  3. Make sure the source is opened in another tab.

• Screenshot if needed:
  None.

• Any related issues:
  None.