Call FetchContext::dispatchWillSendRequest() only if an actual load is started

Call FetchContext::dispatchWillSendRequest() only if an actual load is started

Fixing a regression since [1].
[1] https://codereview.chromium.org/1889973002/

Since [1], FetchContext::dispatchWillSendRequest() was called from
ResourceFetcher::willSendRequest() in the middle of
ResourceFetcher::requestResource(), and might have modified
Resource::m_resourceRequest.
However, if the actual loading is deferred, e.g. when FontResource is not
used, FetchContext::dispatchWillSendRequest() was called without actual
loading, causing stale pending load reports in DevTools.

This CL
- Moves the ResourceFetcher::willSendRequest() call in requestResource() to
  ResourceFetcher::startLoad().
  Now FetchContext::dispatchWillSendRequest() (called from startLoad())
  modifies the local ResourceRequest in startLoad(), not modifying
  ResourceRequest::m_resourceRequest (which is the behavior before [1]).
- Moves setAllowStoredCredentials() call in ResourceFetcher::willSendRequest()
  to its callsites, because this should be done even when the actual loading
  is deferred and should modify Resource::m_resourceRequest.
- Adds a layout test.

BUG=623616, 632580
Review-Url: https://codereview.chromium.org/2184823006
Cr-Commit-Position: refs/heads/master@{#408588}
(cherry picked from commit 1b4209891f51609fe5aa84e13be6dd482ca734c7)

[hiroshige, 2016-08-03 07:38:53]

Committed:
    Review URL: https://codereview.chromium.org/2207833002 .
    
    Cr-Commit-Position: refs/branch-heads/2785@{#480}
    Cr-Branched-From:
68623971be0cfc492a2cb0427d7f478e7b214c24-refs/heads/master@{#403382}