fix(overlay): fixes blurry overlay

[robertmesserle]

• rounds pixel values used in connected-position-strategy
• replaces percent-based offset with calculated pixel values in
  global-position-strategy

closes #932

R: @jelbourn

[jelbourn]

Add comment explaining why we're going off-spec.

[jelbourn]

Need unit tests for this change.

[jelbourn]

@robertmesserle looks like legit test failures

[robertmesserle]

Work still needed:

• The unit tests will need to be refactored using async rather than fakeAsync due to the addition of requestAnimationFrame
• E2E tests are failing on Travis only - I have not been able to recreate this error locally in order to debug.
• We should make the percent conversion a more generic process that supports any values rather than a hard-coded check for -50%.

[philipooo]

I copied these changes to alpha.9 and noticed that the dialog gets not repositioned to the middle of the screen when the content changes (when open). Thanks for the change btw!

[jelbourn]

Replaced by #1962

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