[debugger] Add Stack::EnsureFrames

This is a shortcut for "only sync frames if we haven't already", which
is used in several places, notably for the AsyncTaskTree class which
requires stack frames before doing an already expensive operation.

There should be no functional changes, this is just cleaning up the
APIs.

Test: fx test zxdb_tests && fx test --e2e zxdb_e2e_tests
Cq-Include-Trybots: luci.fuchsia.try:core.x64-debug
Change-Id: I1d3b1c8c7c81481c2e3a4b2264627a0e6a6a6964
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1547860
Commit-Queue: Jacob Rutherford <jruthe@google.com>
Fuchsia-Auto-Submit: Jacob Rutherford <jruthe@google.com>
Reviewed-by: Darren Chan <chandarren@google.com>