[heap profiler] Replace SharedMemoryBuffer with mojo::BigString

BUG=923459

Change-Id: I008e2aa9fea0d5d7d674cb72e141d6d949b463db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1575734
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Primiano Tucci <primiano@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Commit-Queue: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654237}
8 files changed