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This CL fixes the incorrect usage of std::map.

The lookup in the map is using the [] operator which is creating a
default entry when the key is not present.

etienne@ first tried to land this fix but had to revert because some
tests became flaky (which was supposed to happen when we first
associated IPCs with per-frame task runners here
https://chromium-review.googlesource.com/c/chromium/src/+/1526067 ,
but was hidden because of this bug).
Test failure example:
https://ci.chromium.org/p/chromium/builders/ci/Linux%20ChromiumOS%20MSan%20Tests/13652

This is a second try to fix the incorrect map usage.

-- update --

The flaky tests were because of scheduling experiments that set the
background tasks to low priority, and this fix just surfaced the
issue because now tasks are posted to per-frame task runners.
CL to disable the experiments:
https://chromium-review.googlesource.com/c/chromium/src/+/1692577

Now that the experiments are disabled, this memory leak fix should
not make the tests flaky any more.

Bug: 973200, 980523
Change-Id: I1eaf0af67f1bcbd582a510d70174666251b8b4ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1686979
Commit-Queue: Yuzu Saijo <yuzus@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676283}