New SkRTConf macro SK_CONF_TRY_SET: no complaint on missing configuration

SK_CONF_TRY_SET() is like SK_CONF_SET(), but doesn't complain if
confname can't be found.  This is useful if the SK_CONF_DECLARE is
inside a source file whose linkage is dependent on the system.

Internally to the SkRTConf system, SkRTConfRegistry::set() was given
an additional parameter controling wanrings.

A new RuntimeConfig unit test was introduced.  It should run silently.
In the future, it should be expanded to cover all of the SkRTConf

(For example, the images.jpeg.suppressDecoderWarnings variable is
defined and used only in SkImageDecoder_libjpeg.cpp, but on MacOS, we
use Core Graphics via SkImageDecoder_CG.cpp - SkImageDecoder_libjpeg
is never linked in.  The same is true of the Windows Imaging Component
on Windows.)


Review URL:

git-svn-id: 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/tests.gyp b/tests.gyp
index ec5bca0..c5107ac 100644
--- a/tests.gyp
+++ b/tests.gyp
@@ -120,6 +120,7 @@
+        '../tests/RuntimeConfigTest.cpp',