Allow modified inline style into the MPC.

We currently have a special rule saying that if inline style has ever
been modified (i.e., it comes from the parser and has never been
touched by CSSOM or similar), we disallow MPC for the element.
The rationale is that if it's modified once, it will probably be
animated over and over again, filling up the MPC with junk entries.

However, there are situations where inline style is being set once
from CSSOM and then never modified. Furthermore, we already have
a test for property value sets that have been used in the MPC and
then modified (ModifiedSinceHashing()), which will keep out the
elements that keep on being modified. So this test is largely
redundant, and we can remove it.

Speedometer3.1 and MotionMark are both neutral on Pinpoint.

Change-Id: I24330a623a792ac474d99d39ba0a44e3f8db3e0d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7019614
Commit-Queue: Steinar H Gunderson <sesse@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1527440}