Revert of ServiceWorker: Prevent worker script with incorrect MIME type to be registered.

Revert of ServiceWorker: Prevent worker script with incorrect MIME type to be registered. (https://codereview.chromium.org/446323002/)

Reason for revert:
Suspected for breakage on Mac waterfall bots (10.6/10.7)

test ServiceWorkerBrowserTest.ImportsBustMemcache failing since

http://build.chromium.org/p/chromium.mac/builders/Mac10.6%20Tests%20%281%29/builds/54624
http://build.chromium.org/p/chromium.mac/builders/Mac10.7%20Tests%20%281%29/builds/25141

Times out. errors like

[ RUN      ] ServiceWorkerBrowserTest.ImportsBustMemcache
[16435:263:0812/001516:2160051331590:INFO:CONSOLE(13)] "initial unregistration done", source: http://127.0.0.1:58683/service_worker/imports_bust_memcache.html (13)
BrowserTestBase signal handler received SIGTERM. Backtrace:
0   content_browsertests                0x6249bcaa base::debug::StackTrace::StackTrace() + 26
..
11  AppKit                              0x96eeddd6 -[NSApplication nextEventMatchingMask:untilDate:inMode:dequeue:] + 156
12  AppKit                              0x96eb01f3 -[NSApplication run] + 821
..
19  content_browsertests                0x6228bf46 content::TitleWatcher::WaitAndGetTitle() + 118
20  content_browsertests                0x61ed5252 content::ServiceWorkerBrowserTest_ImportsBustMemcache_Test::RunTestOnMainThread() + 530
[649/649] ServiceWorkerBrowserTest.ImportsBustMemcache (TIMED OUT)

Original issue's description:
> ServiceWorker: Prevent worker script with incorrect MIME type to be registered.
> 
> Fix for spec update: https://github.com/slightlyoff/ServiceWorker/issues/380.
> A valid worker script MIME type should be one of below:
> application/x-javascript
> text/javascript
> application/javascript
> 
> BUG=400604
> TEST=https://codereview.chromium.org/448063002/
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=288922

TBR=falken@chromium.org,michaeln@chromium.org,thestig@chromium.org,xiang.long@intel.com
NOTREECHECKS=true
NOTRY=true
BUG=400604
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=288927

[tapted, 2014-08-12 07:38:28]

Created Revert of ServiceWorker: Prevent worker script with incorrect MIME type
to be registered.

[commit-bot: I haz the power, 2014-08-12 07:39:19]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/tapted@chromium.org/459383002/1

[commit-bot: I haz the power, 2014-08-12 07:41:08]

Change committed as 288927