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DeltaSymbolGroup.CountUniqueSymbols() sums separate unique leaf symbol
counts in "before" and "after". Previously this was done via temporary
SymbolGroups and using their CountUniqueSymbols().

As a memory optimization, both temporary SymbolGroups were given Symbol
iterators at construction. However, the |_symbols| member become single-
use as a result. This violates the immutability property of the object,
and causes problem as we plan to process |_symbols| in upcoming work to
add multi-container support.

A short fix is to convert input and make |_symbols| as a list, but this
is opaque, and undoes the optimization above. This CL makes the
following changes instead:

* Assert that SymbolGroup() is given list at construction.
* Replace SymbolGroups.IterUniqueSymbols() with a static generator
  SymbolGroups.IterUnique() that takes an input iterator.
  * All callers undergo minor change.
  * Callers of {Delta}SymbolGroups.CountUniqueSymbol() are unchanged.
* Update DeltaSymbolGroup.CountUniqueSymbols() to use the new interface,
  without needing temporary SymbolGroups. Optimization is kept.

Bug: 1040645
Change-Id: I32c3867fe006a078f9a6ddcf764349b360120e9a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2204304
Commit-Queue: Samuel Huang <huangs@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#769997}