block webgl2 fingerprinting and webgl readPixels

[diracdeltas]

fix #8448

Test Plan:

1. go to https://browserleaks.com/webgl and turn on fingerprinting protection
2. the report hash should be '0C21A6FA2A9BD79DFF6E128FE55094B4'
3. the image hash should be empty
4. all the fields under the triangle image should be empty

• 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).

[diracdeltas]

if you get a 'report hash' other than the value in the test plan, please copy and paste document.querySelector('#webgl-table').innerText. it's not a big deal if it is OS-dependent, but should be fixed if it leaks more information than is in the user-agent string.

[luixxiul]

Test plan works. Confirmed the values become empty with this change. LGTM.

[luixxiul]

CC @bsclifton for confirmation

[diracdeltas]

I think this should be moved into next release because any site could have been evading webgl fingerprinting blocking using the WebGL2RenderingContext API. This API was introduced in Chromium 56, so it's a somewhat recent regression.

[bsclifton]

++