Fetcher tries all proxies when a secondary chunk download error occurs.

This is a fix to issue 18143:

* New test cases for asserting the desired behavior: if a transfer of
  a secondary chunk within a multi-chunk fetch fails, then the fetcher
  needs to retry with other available proxies; it will only fail when no
  additional proxies are available.  The tests ensure both success (one
  of the proxies eventually succeeds) and failure (all proxies fail)
  cases.

* Small fix to LibcurlHttpFetcher to retry with other proxies upon
  failure (error value) of a secondary chunk.

Other changes applied in the course of this fix:

* Massive refactoring of http_fetcher_unittest: substituted template
  specialization in typed test setup with proper subclassing, resulting
  in a safer and more maintainable infrastructure;  extended URLs to
  include all (most) parameters pertaining to test workload, such as
  download size, flakiness, etc.

* Respective changes to test_http_server: it is now much more
  independent of particular kind of tests, and more easily
  parametrizable.  Also, generalized several internal methods for better
  readability and extensibility, such as writing of arbitrary payloads,
  parsing headers,

* Migrated common definitions into http_common.{h,cc} (universal
  HTTP-related stuff) and http_fetcher_unittest.h (shared definitions
  pertaining to unit tests).

* Extended direct proxy resolver to generate a list of (non-) proxies,
  so we can unit test proxy failure.  Also, better logging to improve
  testability.

* Some renaming of classes for better consistency.

BUG=chromium-os:18143
TEST=unit tests

Change-Id: Ib90b53394d7e47184d9953df8fc80348921e8af0
Reviewed-on: https://gerrit.chromium.org/gerrit/12092
Commit-Ready: Gilad Arnold <garnold@chromium.org>
Reviewed-by: Gilad Arnold <garnold@chromium.org>
Tested-by: Gilad Arnold <garnold@chromium.org>
diff --git a/proxy_resolver.h b/proxy_resolver.h
index 53fd920..fb96073 100644
--- a/proxy_resolver.h
+++ b/proxy_resolver.h
@@ -46,16 +46,26 @@
 // Always says to not use a proxy
 class DirectProxyResolver : public ProxyResolver {
  public:
-  DirectProxyResolver() : idle_callback_id_(0) {}
+  DirectProxyResolver() : idle_callback_id_(0), num_proxies_(1) {}
   virtual ~DirectProxyResolver();
   virtual bool GetProxiesForUrl(const std::string& url,
                                 ProxiesResolvedFn callback,
                                 void* data);
 
+  // Set the number of direct (non-) proxies to be returned by resolver.
+  // The default value is 1; higher numbers are currently used in testing.
+  inline void set_num_proxies(size_t num_proxies) {
+    num_proxies_ = num_proxies;
+  }
+
  private:
   // The ID of the idle main loop callback
   guint idle_callback_id_;
 
+  // Number of direct proxies to return on resolved list; currently used for
+  // testing.
+  size_t num_proxies_;
+
   // The MainLoop callback, from here we return to the client.
   void ReturnCallback(ProxiesResolvedFn callback, void* data);
   DISALLOW_COPY_AND_ASSIGN(DirectProxyResolver);