Fix an uninitialized read in a test found by MSan.

A different fix is under discussion at
https://chromium-review.googlesource.com/c/chromium/src/+/1233814
but since there's some disagreement on the approach, let's land
this while the review over there is ongoing, to heal the bot.

Bug: 886629
Change-Id: I80c64f6bf969b5b1edf108d2a46155f2f4252595
Reviewed-on: https://chromium-review.googlesource.com/1232591Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592393}

content/browser/loader/cross_site_document_resource_handler_unittest.cc

@@ -1208,7 +1208,7 @@ class CrossSiteDocumentResourceHandlerTest
 
     // Content-Type header.
     std::string charset;
-    bool had_charset;
+    bool had_charset = false;
     std::string boundary;
     response_headers->AddHeader(std::string("Content-Type: ") +
                                 response_mime_type);