MIPS64: Refine 'Remove unused byte from Map::instance_sizes field.'

MIPS64: Refine 'Remove unused byte from Map::instance_sizes field.'

Port 2027335f1ce44b0c6523eb5e55881e2a4e162815

Original commit message:
Note that there are currently no objects that require a pre-allocated
properties backing store, all such slots are in-object properties from
the begining. Hence {unused + pre_allocated - inobject == 0} holds.

BUG=

Committed: https://crrev.com/9f9f27c3337589306b7f2248ccc472dd576a94c9
Cr-Commit-Position: refs/heads/master@{#29610}

[balazs.kilvady, 2015-07-13 11:46:40]

balazs.kilvady@imgtec.com changed reviewers:
+ akos.palfi@imgtec.com, danno@chromium.org, dusan.milosavljevic@imgtec.com,
mstarzinger@chromium.org, paul.lind@imgtec.com

[Michael Starzinger, 2015-07-13 12:08:17]

LGTM. Thanks. Sorry for not CC'ing you on the original CL. Will do so in the
future.

[balazs.kilvady, 2015-07-13 13:05:49]

The CQ bit was checked by balazs.kilvady@imgtec.com

[commit-bot: I haz the power, 2015-07-13 13:06:02]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1226363003/1

[commit-bot: I haz the power, 2015-07-13 13:36:08]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-13 13:36:25]

Patchset 1 (id:??) landed as
https://crrev.com/9f9f27c3337589306b7f2248ccc472dd576a94c9
Cr-Commit-Position: refs/heads/master@{#29610}