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Pure refactor to support future tree-building features (e.g. filtering symbol
names, filtering symbol types, highlighting...). Two main components:

* Add TreeBuilder, constructs a file tree from an unzipped SizeInfo object (and
  other configs, in the future).
* Split LoadSizeFile (which unzipped the size file and built the tree) into
  file-unzipping and tree-building steps. Benefit: Searching or filtering will
  only rebuild the tree and not perform the costly 1-second decompression step.

Intended use is that caspian_web.cc will create a new TreeBuilder on every
config change.

Bug: 1011921
Change-Id: I73f37bacc6657b8c33e4bdcd873e97a80f27a10b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1884828
Commit-Queue: Jasper Chapman-Black <jaspercb@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710480}