Enable chrome://discards on Linux.

Tab manager was enabled on Linux a while ago but  chrome://discards was never made available on that platform. This CL fixes that.

Note that automatic discarding is still not available on Linux due to the lack of memory pressure subsystem on that platform.

BUG=551995,642327,650158

Review-Url: https://codereview.chromium.org/2453453002
Cr-Commit-Position: refs/heads/master@{#428173}
4 files changed