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GpuChannel currently calculates current memory usage by iterating over
all stubs and summing their individual memory usage. This results in
double counting as a given stub may share its memory tracker with other
stubs, if they are in a share group.

To avoid double counting, we now iterate over stubs to collect a set of
unique MemoryTrackers, then sum the memory usage of each unique tracker.

BUG=512535

Review URL: https://codereview.chromium.org/1244843004

Cr-Commit-Position: refs/heads/master@{#339746}