[vm/ios/timeline] Enable cpu time reporting on ios.
[vm/ios/timeline] Enable cpu time reporting on ios.

This was disabled previously due to inconsistent cpu time reading reported by the ios.
It seems to be working correctly now.

TEST=verified flutter application cpu timeline numbers in observatory on physical ios device

Change-Id: Id97a550bbbc96a32b5c2ccf59ca9805ea8cb9ec4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/200703
Commit-Queue: Alexander Aprelev <dfb74c35a357c2059971a4bf7fe26b6a7c18085d@google.com>
Reviewed-by: Ryan Macnak <d738b7f001baf9789095d7dc549cff6de60b011d@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk