Add `devtools:` as a secure context.

We trust `devtools:` in the same way we trust `chrome:`. We should
consider these resources to be secure contexts, as this will enable
us to harden them with mechanisms like Trusted Types that are only
available in such contexts.

Bug: 1070526
Change-Id: I0ac60c642e1afd3f81d669d60a399a94ab0f0751
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2145882Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759163}

content/common/url_schemes.cc

@@ -69,6 +69,7 @@ void RegisterContentSchemes() {
   for (auto& scheme : schemes.referrer_schemes)
     url::AddReferrerScheme(scheme.c_str(), url::SCHEME_WITH_HOST);
 
+  schemes.secure_schemes.push_back(kChromeDevToolsScheme);
   schemes.secure_schemes.push_back(kChromeUIScheme);
   schemes.secure_schemes.push_back(kChromeUIUntrustedScheme);
   schemes.secure_schemes.push_back(kChromeErrorScheme);