Update FileSystemSyncAccessHandle async methods to sync

[dslee414]

Per discussions from https://github.com/whatwg/fs/issues/7 and https://github.com/whatwg/fs/issues/28, this PR updates FileSystemSyncAccessHandle getSize(), truncate(), flush() and close() asynchronous methods to synchronous.

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

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