[ExploreSites] Update experiment header to follow GFE headers policy.

The current header is being stripped out by GFE, and therefore doesn't
affect the response from the server. Without this current change, we
cannot conduct any server-side experiments based on the Finch
parameter.

Read more about the policy here:
go/gfe-headers

Server-side equivalent:
cl/267619352

Bug: 1001537
Change-Id: I13b2a177bcc4527bb6dd294954da83cedae476b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1789665
Commit-Queue: Sebastien Lalancette <seblalancette@google.com>
Reviewed-by: Justin DeWitt <dewittj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694290}

chrome/browser/android/explore_sites/explore_sites_fetcher.cc

@@ -166,7 +166,7 @@ void ExploreSitesFetcher::Start() {
           kExploreSitesHeadersExperimentParameterName);
 
   if (!tag.empty()) {
-    resource_request->headers.SetHeader("X-Google-Chrome-Experiment-Tag", tag);
+    resource_request->headers.SetHeader("X-Goog-Chrome-Experiment-Tag", tag);
   }
 
   url_loader_ = network::SimpleURLLoader::Create(std::move(resource_request),

chrome/browser/android/explore_sites/explore_sites_fetcher_unittest.cc

@@ -356,7 +356,7 @@ TEST_F(ExploreSitesFetcherTest, TestHeaders) {
   EXPECT_EQ(kAcceptLanguages, languages);
 
   // The finch header should not be set since the experiment is not on.
-  success = headers.HasHeader("X-Google-Chrome-Experiment-Tag");
+  success = headers.HasHeader("X-Goog-Chrome-Experiment-Tag");
   EXPECT_FALSE(success);
 }
 
@@ -372,7 +372,7 @@ TEST_F(ExploreSitesFetcherTest, TestFinchHeader) {
   std::string header_text;
   bool success;
 
-  success = headers.GetHeader("X-Google-Chrome-Experiment-Tag", &header_text);
+  success = headers.GetHeader("X-Goog-Chrome-Experiment-Tag", &header_text);
   EXPECT_EQ(std::string(kExperimentData), header_text);
 }