test-webkitpy fails on Mac without iphoneos SDK

test-webkitpy fails on Mac without iphoneos SDK
https://bugs.webkit.org/show_bug.cgi?id=141255
<rdar://problem/19619691>

Reviewed by David Kilzer and Alexey Proskuryakov.

Fixes an issue where test-webkitpy fails on a Mac without the iOS SDK. We should
should not require the iphoneos SDK to be installed to run the webkitpy tests.

* Scripts/webkitpy/common/system/platforminfo.py:
(PlatformInfo.xcode_sdk_version): Added; return the version of the specified SDK, if
installed. We take advantage of the behavior that xcrun --show-sdk-version only writes
to standard output if the SDK is installed to return the empty string if the SDK is
not installed.
* Scripts/webkitpy/common/system/platforminfo_mock.py:
(MockPlatformInfo.xcode_sdk_version): Added.
* Scripts/webkitpy/port/ios.py:
(IOSPort.determine_full_port_name): Modified to call PlatformInfo.xcode_sdk_version()
to get the version of the iphoneos SDK, if installed.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@179622 268f45cc-cd09-0410-ab3c-d52691b4dbfc