Revert of Re-land: base: Enable browser-wide discardable memory on Linux, CrOS and Windows.

Revert of Re-land: base: Enable browser-wide discardable memory on Linux, CrOS and Windows. (patchset #1 id:1 of https://codereview.chromium.org/793443003/)

Reason for revert:
Caused random crashes on GPU bots with this assertion failure:
WebglConformance.conformance_ogles_GL_log_log_001_to_008:
Error installing extension 'v8/gc'.
[3060:3996:0218/193241:FATAL:discardable_shared_memory_heap.cc(205)] Check failed: spans_.find(span->start_ + span->length_ - 1) != spans_.end().

Original issue's description:
> Re-land: base: Enable browser-wide discardable memory on Linux, CrOS and Windows.
>
> This makes SHMEM implementation of discardable memory preferred
> over EMULATED implementation. This effectively makes SHMEM the
> implementation used by default on Linux, CrOS and Windows.
>
> SHMEM implementation of discardable memory gives the browser
> process control over the total amount of discardable memory used
> and allows us to enforce a global limit of 512MB across all
> renderers.
>
> BUG=429415, 429416
>
> Committed: https://crrev.com/2f7a8e80b83aa581aa6de86117a692093d16c217
> Cr-Commit-Position: refs/heads/master@{#316977}

TBR=avi@chromium.org,danakj@chromium.org,reveman@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=429415, 429416
Committed: https://crrev.com/e8360108c83458621520fc41aa00359bc90955a5
Cr-Commit-Position: refs/heads/master@{#316997}

[Ken Russell (switch to Gerrit), 2015-02-19 06:19:25]

Created Revert of Re-land: base: Enable browser-wide discardable memory on
Linux, CrOS and Windows.

[commit-bot: I haz the power, 2015-02-19 06:20:39]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/917023003/1

[commit-bot: I haz the power, 2015-02-19 06:21:33]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-19 06:22:02]

Patchset 1 (id:??) landed as
https://crrev.com/e8360108c83458621520fc41aa00359bc90955a5
Cr-Commit-Position: refs/heads/master@{#316997}