Add optional sw generated path coverage mask caching

BUG=skia:
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Review-Url: https://codereview.chromium.org/2335343008
diff --git a/src/gpu/GrContext.cpp b/src/gpu/GrContext.cpp
index fbb59a3..6cec1da 100644
--- a/src/gpu/GrContext.cpp
+++ b/src/gpu/GrContext.cpp
@@ -97,6 +97,8 @@
     dtOptions.fMaxBatchLookahead = options.fMaxBatchLookahead;
     GrPathRendererChain::Options prcOptions;
     prcOptions.fDisableDistanceFieldRenderer = options.fDisableDistanceFieldPaths;
+    prcOptions.fAllowPathMaskCaching = options.fAllowPathMaskCaching;
+    prcOptions.fDisableAllPathRenderers = options.fForceSWPathMasks;
     fDrawingManager.reset(new GrDrawingManager(this, dtOptions, prcOptions, options.fImmediateMode,
                                                &fSingleOwner));