linux: remove use_system_libxslt, instead use xml flag
linux: remove use_system_libxslt, instead use xml flag

We have weird crashes when system_libxslt != system_libxml.
Nobody has diagnosed them.  We should just use one flag for both
so people stop reporting problems with it.

BUG=42487

Review URL: http://codereview.chromium.org/1695019

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@45766 0039d316-1c4b-4281-b951-d872f2087c98

ropik/chromium,ropik/chromium,adobe/chromium,Crystalnix/house-of-life-chromium,Crystalnix/house-of-life-chromium,Crystalnix/house-of-life-chromium,ropik/chromium,yitian134/chromium,gavinp/chromium,yitian134/chromium,adobe/chromium,yitian134/chromium,Crystalnix/house-of-life-chromium,Crystalnix/house-of-life-chromium,ropik/chromium,adobe/chromium,gavinp/chromium,ropik/chromium,yitian134/chromium,yitian134/chromium,yitian134/chromium,adobe/chromium,yitian134/chromium,gavinp/chromium,adobe/chromium,adobe/chromium,adobe/chromium,gavinp/chromium,yitian134/chromium,gavinp/chromium,Crystalnix/house-of-life-chromium,yitian134/chromium,adobe/chromium,Crystalnix/house-of-life-chromium,gavinp/chromium,yitian134/chromium,gavinp/chromium,ropik/chromium,adobe/chromium,Crystalnix/house-of-life-chromium,ropik/chromium,adobe/chromium,Crystalnix/house-of-life-chromium,ropik/chromium,adobe/chromium,gavinp/chromium,gavinp/chromium,gavinp/chromium,Crystalnix/house-of-life-chromium,Crystalnix/house-of-life-chromium,ropik/chromium