[CMake] Miscellaneous issues in WebKitFeatures.cmake

[CMake] Miscellaneous issues in WebKitFeatures.cmake
https://bugs.webkit.org/show_bug.cgi?id=143636

Reviewed by Martin Robinson.

Rename _WEBKIT_AVAILABLE_OPTIONS_INITIALVALUE_ variables to
_WEBKIT_AVAILABLE_OPTIONS_INITIAL_VALUE_

Rename _WEBKIT_AVAILABLE_OPTIONS_ISPUBLIC_ variables to
_WEBKIT_AVAILABLE_OPTIONS_IS_PUBLIC_

Rename _SHOULD_PRINT_POINTS to _SHOULD_PRINT_DOTS

Update USE_SYSTEM_MALLOC description to not mention TCmalloc

Fix ENABLE_TOUCH_SLIDER so that it can be used

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* Source/cmake/WebKitFeatures.cmake:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@182664 268f45cc-cd09-0410-ab3c-d52691b4dbfc