GetPrimarySolutionPath() is used by GetBuildtoolsPath() to locate
the chromium/src directory, its return value shouldn't include
'buildtools', since GetBuildtoolsPath() will append another one to
it.

Introduced by https://codereview.chromium.org/933383002

Review URL: https://codereview.chromium.org/1072973003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@294894 0039d316-1c4b-4281-b951-d872f2087c98