Revert of Reland #1: "base: Implement GetCurrentThreadPriority." (patchset #2 id:20001 of https://codereview.chromium.org/2215513002/ )

Reason for revert:
Still causes startup regressions:

https://bugs.chromium.org/p/chromium/issues/detail?id=635464#c4

Original issue's description:
> Reland #1: "base: Implement GetCurrentThreadPriority."
>
> The Chrome thread priority is saved in the thread dictionary during
> SetCurrentThreadPriority(). This seemed a better approach than using
> thread_policy_get(), since there are 4 Chrome priorities which won't cleanly map
> to thread flavors.
>
> BUG=601270
>
> Committed: https://crrev.com/23d10a36f2e815edd610098a65e5890f279d2183
> Cr-Commit-Position: refs/heads/master@{#410163}

TBR=mark@chromium.org,gab@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=601270

Review-Url: https://codereview.chromium.org/2221083002
Cr-Commit-Position: refs/heads/master@{#410390}
3 files changed