Revert "Don't sniff HTML from documents delivered via the file protocol"

This reverts commit 3519e867dc606437f804561f889d7ed95b95876a.

Reason for revert: crbug.com/786150. Application compatibility for Android WebView applications means we need to allow sniffing on that platform. 

Original change's description:
> Don't sniff HTML from documents delivered via the file protocol
> 
> To reduce attack surface, Chrome should not MIME-sniff to text/html for
> any document delivered via the file protocol. This change only impacts
> the file protocol (documents served via HTTP/HTTPS/etc are unaffected).
> 
> Bug: 777737
> Cq-Include-Trybots: master.tryserver.chromium.android:android_cronet_tester;master.tryserver.chromium.mac:ios-simulator-cronet
> Change-Id: I7086454356b8d2d092be9e1bca0f5ff6dd3b62c0
> Reviewed-on: https://chromium-review.googlesource.com/751402
> Reviewed-by: Ben Wells <benwells@chromium.org>
> Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
> Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
> Reviewed-by: Asanka Herath <asanka@chromium.org>
> Reviewed-by: Matt Menke <mmenke@chromium.org>
> Commit-Queue: Eric Lawrence <elawrence@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#514372}

TBR=achuith@chromium.org,benwells@chromium.org,mmenke@chromium.org,sdefresne@chromium.org,asanka@chromium.org,elawrence@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 777737
Change-Id: I864ae060ce3277d41ea257ae75e0b80c51f3ea98
Cq-Include-Trybots: master.tryserver.chromium.android:android_cronet_tester;master.tryserver.chromium.mac:ios-simulator-cronet
Reviewed-on: https://chromium-review.googlesource.com/790790
Reviewed-by: Eric Lawrence <elawrence@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Eric Lawrence <elawrence@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519347}
  void JSExpect(const std::string& expression) {
    bool result;
    EXPECT_TRUE(content::ExecuteScriptAndExtractBool(
        browser()->tab_strip_model()->GetActiveWebContents(),
        "window.domAutomationController.send(!!(" + expression + "));",
        &result));
    EXPECT_TRUE(result) << expression;
  }

  void JSExpect(const std::string& expression) {
    bool result;
    EXPECT_TRUE(content::ExecuteScriptAndExtractBool(
        browser()->tab_strip_model()->GetActiveWebContents(),
        "window.domAutomationController.send(!!(" + expression + "));",
        &result));
    EXPECT_TRUE(result) << expression;
  }
