Gradle: should not use CONF to determine the build type for native webkit build

With the fix for RT-31500 decoupled building Release (optimized) versus Debug for Java code and native code. The CONF flag now sets separate "IS_DEBUG" and "IS_DEBUG_NATIVE" flags that are used to tell the rest of the build script what type of build it is (rather than using CONF directly which some code is still doing). As a result, the following need to change:

            println "Building Webkit configuration /$CONF/ into $webkitOutputDir"
                commandLine("perl", "Tools/Scripts/set-webkit-configuration", "--$CONF")
                from "$webkitOutputDir/$CONF/lib/${library('jfxwebkit')}"
                from "$webkitOutputDir/$CONF/lib/${library('DumpRenderTreeJava')}"
            def gensrcDir = "$webkitOutputDir/$CONF/WebCore/generated/java"