Remove warnings from MemoryPurger

Remove warnings from MemoryPurger

Refactoring of MemoryPurger class. Warnings, unused includes and dead
code have been removed.

Erased code contained helper class calling non-existent memory purging
methods of ProxyService. MemomoryPurger was marked in previous commits
as obsolete https://code.google.com/p/chromium/issues/detail?id=350455.
MemoryPurger has not been removed from code base due to existing references
to its interface. Currently only EWebContext::NotifyLowMemory invokes
MemoryPurger methods.

Original beta/m47 patch: http://165.213.202.130/gerrit/#/c/104842/

Bug: http://107.108.218.239/bugzilla/show_bug.cgi?id=4822

Reviewed by: a1.gomes

Change-Id: Ib77e5d2983c38fa1281f37902370d2d1ad387748
Signed-off-by: Leszek Syroka <l.syroka@partner.samsung.com>
(cherry picked from commit f776a5291d3af1bea61497155cb3724cf0ef3aa9)