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Plumb the internal display id to populate the Screen.internal attribute.
Follows the primary display id and Screen.primary attribute pattern.

This requires the ScreenEnumeration experimental flag to be enabled:
  chrome://flags#enable-experimental-web-platform-features OR
  $ chrome --enable-blink-features=ScreenEnumeration

Test with this demo:
  https://github.com/michaelwasserman/window-placement-demo

Bug: 994889
Test: getScreens()[i].internal returns an accurate value.
Change-Id: I71ed5eb8b164eff6f2f5d93a7a400e69df219490
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2015555
Commit-Queue: Michael Wasserman <msw@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734550}