This patch introduces the leak detector's integration layer,
which gathers alloc/free info and passes it to the leak
detection logic in LeakDetectorImpl.

It registers callbacks with the allocator, which are called
during alloc and free. These callbacks intercept and sample
the alloc and free info.

It also provides a notification interface for external
objects to receive leak reports.

See leak_detector.cc for more info.

BUG=chromium:382705
TEST=build successfully, run components_unittests

Review URL: https://codereview.chromium.org/1665553002

Cr-Commit-Position: refs/heads/master@{#381293}