update_engine: fixed warnings from cpplint

Fixed all the cpplint warnings in update engine.

BUG=None
TEST=Unit tests still pass.

Change-Id: I285ae858eec8abe0b26ff203b99a42a200ceb71c
Reviewed-on: https://chromium-review.googlesource.com/204027
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>
diff --git a/multi_range_http_fetcher.h b/multi_range_http_fetcher.h
index 8b5ace8..800c7fd 100644
--- a/multi_range_http_fetcher.h
+++ b/multi_range_http_fetcher.h
@@ -6,6 +6,7 @@
 #define CHROMEOS_PLATFORM_UPDATE_ENGINE_MULTI_RANGE_HTTP_FETCHER_H_
 
 #include <deque>
+#include <string>
 #include <utility>
 #include <vector>
 
@@ -106,7 +107,7 @@
   class Range {
    public:
     Range(off_t offset, size_t length) : offset_(offset), length_(length) {}
-    Range(off_t offset) : offset_(offset), length_(0) {}
+    explicit Range(off_t offset) : offset_(offset), length_(0) {}
 
     inline off_t offset() const { return offset_; }
     inline size_t length() const { return length_; }