update_engine: Update libchrome APIs to r369476

The new libchrome has been ported from Chromium and some APIs have
changed. Make necessary changes at call sites.

(cherry picked from commit 0103c36caa2e38e034e0d22185736b9ccfb35c58)

Change-Id: I3dedd8b3f6e92ce8d3eeef99a76ad876c29db304
diff --git a/chrome_browser_proxy_resolver.cc b/chrome_browser_proxy_resolver.cc
index 4971d74..da57e1d 100644
--- a/chrome_browser_proxy_resolver.cc
+++ b/chrome_browser_proxy_resolver.cc
@@ -162,13 +162,12 @@
     // Start by finding the first space (if any).
     string::iterator space;
     for (space = token.begin(); space != token.end(); ++space) {
-      if (IsAsciiWhitespace(*space)) {
+      if (base::IsAsciiWhitespace(*space)) {
         break;
       }
     }
 
-    string scheme = string(token.begin(), space);
-    base::StringToLowerASCII(&scheme);
+    string scheme = base::ToLowerASCII(string(token.begin(), space));
     // Chrome uses "socks" to mean socks4 and "proxy" to mean http.
     if (scheme == "socks")
       scheme += "4";