skiaserve: Change alpha setting from GET to POST.

tools/skiaserve/skiaserve.cpp

Index: tools/skiaserve/skiaserve.cpp
diff --git a/tools/skiaserve/skiaserve.cpp b/tools/skiaserve/skiaserve.cpp
index d8430d7853181a9e12f2dfba103ea7ed0e88662d..09819cf22d0b3aad4fc292fd6e8ad12a9ef1b2ec 100644
--- a/tools/skiaserve/skiaserve.cpp
+++ b/tools/skiaserve/skiaserve.cpp
@@ -299,7 +299,7 @@ class ClipAlphaHandler : public UrlHandler {
 public:
     bool canHandle(const char* method, const char* url) override {
         static const char* kBasePath = "/clipAlpha/";
-        return 0 == strcmp(method, MHD_HTTP_METHOD_GET) &&
+        return 0 == strcmp(method, MHD_HTTP_METHOD_POST) &&
                0 == strncmp(url, kBasePath, strlen(kBasePath));
     }