Avoid printing draw target info to stderr while running unit tests

Change draw target dump function to return a SkString. Clients can do
whatever they want with the string.

BUG=skia:1837
R=caryclark@google.com, bsalomon@google.com

Author: kkinnunen@nvidia.com

Review URL: https://codereview.chromium.org/72353003

git-svn-id: http://skia.googlecode.com/svn/trunk@12340 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/gpu/gl/GrGpuGL.cpp b/src/gpu/gl/GrGpuGL.cpp
index 0d1ea8e..9cf39b6 100644
--- a/src/gpu/gl/GrGpuGL.cpp
+++ b/src/gpu/gl/GrGpuGL.cpp
@@ -140,7 +140,7 @@
         GrPrintf("------ EXTENSIONS\n");
         ctx.info().extensions().print();
         GrPrintf("\n");
-        ctx.info().caps()->print();
+        GrPrintf(ctx.info().caps()->dump().c_str());
     }
 
     fProgramCache = SkNEW_ARGS(ProgramCache, (this));