Simplify partitionReallocGeneric() implementation

Using the new partitionAllocActualSize() and partitionAllocGetSize()
the implementation of partitionReallocGeneric() can be made quite a
bit simpler and more high-level.

R=cevans@chromium.org
BUG=

Review URL: https://codereview.chromium.org/180293003

git-svn-id: svn://svn.chromium.org/blink/trunk@167997 bbb929c8-8fbe-4397-9dbb-9b2b20218538