fix(dialog): Cancel open's rAF when close is called

[kfranqueiro]

This fixes a bug in Dialog when open() and close() are called on the same frame, where the dialog is visually open but internally its state says it's closed, which then makes close() do nothing on subsequent calls.

I discovered this in the wake of Abhinay discovering an issue with the current snackbar JS PR (#4081), after he fixed a similar type of issue for Drawer (#4078). We're probably going to model snackbar off of dialog, so I wanted to make sure that worked.

To repro the original issue using this branch:

1. Comment out the fix in #close
2. npm start
3. Open http://localhost:8080/spec/mdc-dialog/classes/baseline-alert.html
4. Open the console and run:

   var dialog = mdc.testFixture.dialogs[0];
   dialog.open();
   dialog.close();

(This issue only happens on the same frame, due to neglecting to cancel #open's rAF.)

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@2ae6335). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #4087   +/-   ##
=========================================
  Coverage          ?   98.67%           
=========================================
  Files             ?      126           
  Lines             ?     5603           
  Branches          ?      746           
=========================================
  Hits              ?     5529           
  Misses            ?       74           
  Partials          ?        0

Impacted Files	Coverage Δ
packages/mdc-dialog/foundation.js	100% <100%> (ø)

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[mdc-web-bot]

All 663 screenshot tests passed for commit ff2aa07 vs. master! 💯🎉

[mdc-web-bot]

All 663 screenshot tests passed for commit bc933e4 vs. master! 💯🎉

[acdvorak]

LGTM!

Confirmed that this also works in #4081. Thanks Ken!

[mdc-web-bot]

All 663 screenshot tests passed for commit 205e90b vs. master! 💯🎉

[mdc-web-bot]

All 663 screenshot tests passed for commit 10cbc71 vs. master! 💯🎉