Adapt ui/libgtkui to jumbo compilation and add jumbo support

This moves a duplicated GetGtkSettingsStringProperty to
gtk_util.cc and renames two EnsureMethodsLoaded so that they
don't clash when jumbo compiled.

Jumbo is a Chromium implementation of a unity build system, a
build where many cc files are compiled in a single unit
(translation unit). This can make compilations much faster but
adds a bit of latency and reduces the parallelism.

Jumbo in this code saves about 1 CPU minute (which is not much)
so it's mostly for completion and because I've had the patch
for a long time.

Bug: 803406
Change-Id: Ib3b855d81ea45743568f8b247731f70d8bc659d6
Reviewed-on: https://chromium-review.googlesource.com/c/1446218
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#627873}
7 files changed