[Android WebView] Tweak gpu memory for low end devices

For low end and low resolution devices, allocation steps of 10M is too
coarse, so lower it down to 5M. This also affects small WebViews.

Also lower the multiplier to 10 to be more conservative.

BUG=
NOTRY=true

Review URL: https://codereview.chromium.org/24927002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225736 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225736