[webkitpy] PlatformInfo should have default argument for casual use

[webkitpy] PlatformInfo should have default argument for casual use
https://bugs.webkit.org/show_bug.cgi?id=180827

Patch by Basuke Suzuki <Basuke.Suzuki@sony.com> on 2018-01-09
Reviewed by David Kilzer.

PlatformInfo should be the only way to check platform specific thing instead
of sys.platform or other various checks. If there is something you want to check,
it should be placed in PlatformInfo.

Currently to instantiate PlatformInfo, it requires arguments, which is usually
sys, platform and Executive(). They are straight forward and should be handled
by default arguments. Then we can instantiate PlatformInfo more casual.

* Scripts/webkitpy/common/system/platforminfo.py:
(PlatformInfo): Use relative module path import
(PlatformInfo.__init__):
(PlatformInfo.executive): Lazy instantiation of Executive()
(PlatformInfo.total_bytes_memory): Accessing via above property
(PlatformInfo.xcode_sdk_version): Ditto
(PlatformInfo.xcode_simctl_list): Ditto
(PlatformInfo.xcode_version): Ditto
(PlatformInfo._win_version_str): Ditto
* Scripts/webkitpy/common/system/systemhost.py:
(SystemHost.__init__):
* Scripts/webkitpy/common/system/user.py:
(User.__init__):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@226652 268f45cc-cd09-0410-ab3c-d52691b4dbfc