Previously, the sampling heap profiler stored a list of samples and then
built a tree representation when the profile was queried by calling
GetAllocationProfile. This change reduces duplication by removing stacks
from all samples. Also, less information is stored in the tree
maintained by the profiler and remaining information (script name, line
no, etc) is resolved when a profile is requested.

BUG=

Review URL: https://codereview.chromium.org/1697903002

Cr-Commit-Position: refs/heads/master@{#34119}