Resolve ambiguous base::File reference

When libchrome includes libbase headers, the compiler looks for
base::File in ::android::base rather than ::base, because we're
inside the android namespace.

Bug:  28117776
Change-Id: I3b423845bfe5bacd1519a8973acf5243e665c315
Test: Compiles in that situation.

webservd/binder_request.cc

diff --git a/webservd/binder_request.cc b/webservd/binder_request.cc
index e2a9a8e..96c0a86 100644
--- a/webservd/binder_request.cc
+++ b/webservd/binder_request.cc
@@ -107,7 +107,7 @@
     headers_map.emplace_back(*it, *(it + 1));
   }
 
-  base::File file = request_->Complete(status_code, headers_map, data_size);
+  ::base::File file = request_->Complete(status_code, headers_map, data_size);
 
   if (!file.IsValid()) {
     // We've already handled this request