Update FileSystemSyncAccessHandle async methods to sync

[dslee414]

Per discussions from #7 and #28, this PR updates FileSystemSyncAccessHandle getSize(), truncate(), flush() and close() asynchronous methods to synchronous.

• At least two implementers are interested (and none opposed):
  • Chrome is in the process of shipping
  • Mozilla is interested
  • None opposed
• Tests are written and can be reviewed and commented upon at:
  • Tests are in draft and to be merged upon approval
• Implementation bugs are filed:
  • Chrome: https://bugs.chromium.org/p/chromium/issues/detail?id=1338340
  • Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=1797579
  • Safari: https://bugs.webkit.org/show_bug.cgi?id=247071

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[a-sully]

LGTM! @annevk please feel free to merge this if it looks good to you. Thanks!

[annevk]

Before merging it's good practice to have all the checkboxes in OP checked, but also have the items actually completed and not as TBA. 😊 Could someone please file the remaining implementation bugs?

[a-sully]

Filed https://bugs.webkit.org/show_bug.cgi?id=247071 and https://bugzilla.mozilla.org/show_bug.cgi?id=1797579