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This memory limit is a lowest-common-denominator type value that should
work for more-or-less all platforms. Currently this value causes
impl-side painting to increase the size of the renderer process resident
memory, which is causing memory regressions on the perf bots.

But, having enough memory for 2 frames seems excessive as the baseline
since in the common case we need to upload resources for the next
compositor deadline that will be presented at the next vsync. So, remove
the 2x multiplier from this constant value.

R=reveman
BUG=407193

Review URL: https://codereview.chromium.org/522023002

Cr-Commit-Position: refs/heads/master@{#293001}