Let jumbo use root_build_dir relative paths

Up until now include paths have been relative to the location
of the generated jumbo files. That can make paths rather long 
with many "../../.." in them.

In some circumstances, the paths are now too long for some
Win32 APIs (260 chars) so let us try with 
root_build_dir relative paths instead.

Bug: 778627
Change-Id: Ic7202789f75caf5134b38b174cb5b24a217d0035
Reviewed-on: https://chromium-review.googlesource.com/737836
Commit-Queue: Daniel Bratell <bratell@opera.com>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#511920}