Set GeometryBufferMapThreshold defaults

TBR=bsalomon@google.com
BUG=skia:

Review URL: https://codereview.chromium.org/1161543003

include/gpu/GrCaps.h

@@ -203,6 +203,7 @@ public:
     }
 
     size_t geometryBufferMapThreshold() const {
+        SkASSERT(fGeometryBufferMapThreshold >= 0);
         return fGeometryBufferMapThreshold;
     }
 
@@ -229,6 +230,7 @@ protected:
 
     BlendEquationSupport fBlendEquationSupport;
     uint32_t fMapBufferFlags;
+    int fGeometryBufferMapThreshold;
 
     int fMaxRenderTargetSize;
     int fMaxTextureSize;
@@ -242,7 +244,6 @@ protected:
 private:
     bool fSupressPrints : 1;
     bool fDrawPathMasksToCompressedTextureSupport : 1;
-    size_t fGeometryBufferMapThreshold;
 
     typedef SkRefCnt INHERITED;
 };

include/gpu/GrContextOptions.h

@@ -17,7 +17,7 @@ struct GrContextOptions {
         , fMaxTextureSizeOverride(SK_MaxS32)
         , fMinTextureSizeOverride(0)
         , fSuppressDualSourceBlending(false)
-        , fGeometryBufferMapThreshold(1 << 15) {}
+        , fGeometryBufferMapThreshold(-1) {}
 
     // EXPERIMENTAL
     // May be removed in the future, or may become standard depending
@@ -35,11 +35,10 @@ struct GrContextOptions {
     int  fMinTextureSizeOverride;
     bool fSuppressDualSourceBlending;
 
-    /** fGeometryBufferMapThreshold gives a threshold (in bytes) for when Gr should
-        map a GrGeometryBuffer to update its contents. It will use map() if the
-        size of the updated region is greater than the threshold. Otherwise it will
-        use updateData(). */
-    size_t  fGeometryBufferMapThreshold;
+    /** the threshold in bytes above which we will use a buffer mapping API to map vertex and index
+        buffers to CPU memory in order to update them.  A value of -1 means the GrContext should
+        deduce the optimal value for this platform. */
+    int  fGeometryBufferMapThreshold;
 };
 
 #endif

src/gpu/gl/GrGLCaps.cpp

@@ -271,6 +271,12 @@ void GrGLCaps::init(const GrGLContextInfo& ctxInfo, const GrGLInterface* gli) {
         }
     }
 
+    // On many GPUs, map memory is very expensive, so we effectively disable it here by setting the
+    // threshold to the maximum unless the client gives us a hint that map memory is cheap.
+    if (fGeometryBufferMapThreshold < 0) {
+        fGeometryBufferMapThreshold = ctxInfo.isChromium() ? 0 : SK_MaxS32;
+    }
+
     if (kGL_GrGLStandard == standard) {
         SkASSERT(ctxInfo.version() >= GR_GL_VER(2,0) ||
                  ctxInfo.hasExtension("GL_ARB_texture_non_power_of_two"));