[core] Move parseCacheControl() up in the class hierarchy

Avoid duplicating it on all the ports.

src/mbgl/storage/http_request_base.cpp

@@ -0,0 +1,22 @@
+#include <mbgl/storage/http_request_base.hpp>
+
+#include <mbgl/util/chrono.hpp>
+
+namespace mbgl {
+
+int64_t HTTPRequestBase::parseCacheControl(const char *value) {
+    if (value) {
+        unsigned long long seconds = 0;
+        // TODO: cache-control may contain other information as well:
+        // http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.9
+        if (std::sscanf(value, "max-age=%llu", &seconds) == 1) {
+            return std::chrono::duration_cast<std::chrono::seconds>(
+                       std::chrono::system_clock::now().time_since_epoch()).count() +
+                   seconds;
+        }
+    }
+
+    return 0;
+}
+
+}