[ VM / Profiler ] Add --sample-buffer-duration flag to specify sample buffer size

This flag consumes a time in seconds and calculates the correct capacity
for the sample buffer so it can contain at least that many seconds worth
of samples based on the maximum number of samples that can be chained to
represent a stack and the sampling rate.

Providing a value to this flag will likely greatly increase memory
consumption, so a warning message is printed which contains the
information used to calculate the buffer capacity as well as the size of
the resulting buffer in bytes. Example output:

** WARNING ** Custom sample buffer size provided via --sample-buffer-duration
The sample buffer can hold at least 120s worth of samples with stacks depths of up to 128, collected at a sample rate of 20000Hz.
The resulting sample buffer size is 3532800000 bytes.

Fixes https://github.com/dart-lang/sdk/issues/37924

Change-Id: I58aff6da268b9a6b02258021d2be8bf4f80ae999
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/117208
Reviewed-by: Siva Annamalai <asiva@google.com>
Commit-Queue: Ben Konyi <bkonyi@google.com>