Re-landing https://chromiumcodereview.appspot.com/11587012: Introduce mac_want_real_dsym GYP flag.

Re-landing https://chromiumcodereview.appspot.com/11587012: Introduce mac_want_real_dsym GYP flag.
By default, fake .dSYM files
will be generated for regular builds, and dsymutils will be used to create .dSYMs for ASan builds.
mac_want_real_dsym can be used to override this.

mac_real_dsym can't be set directly, because it's not a % variable.
According to https://codereview.chromium.org/113999, it can't be made a
% variable now, because of a bug in % handling in GYP that leads to
mac_real_dsym being unconditionally set to 1 for some targets.

BUG=148383
TBR=thakis
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=173896

[Alexander Potapenko, 2012-12-19 11:08:48]

Nico, TBR