Revert of XMLHttpRequest::overrideMimeType should throw InvalidStateError

Revert of XMLHttpRequest::overrideMimeType should throw InvalidStateError (patchset #4 of https://codereview.chromium.org/478223002/)

Reason for revert:
It looks like this broke a number of Chromium-side browser tests: See http://build.chromium.org/p/chromium.webkit/builders/Linux%20Tests%20%28dbg%29/builds/3608 for example. http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%40ToT%20Blink&testType=browser_tests&tests=ExtensionBrowserTest.RSSParseFeedInvalidFeed1,ExtensionBrowserTest.RSSParseFeedInvalidFeed2,ExtensionBrowserTest.RSSParseFeedInvalidFeed3,ExtensionBrowserTest.RSSParseFeedInvalidFeed4,ExtensionBrowserTest.RSSParseFeedValidFeed0,ExtensionBrowserTest.RSSParseFeedValidFeed2,ExtensionBrowserTest.RSSParseFeedValidFeed3,ExtensionBrowserTest.RSSParseFeedValidFeed4,ExtensionBrowserTest.RSSParseFeedValidFeed5,ExtensionBrowserTest.RSSParseFeedValidFeed6,ExtensionBrowserTest.RSSParseFeedValidFeedNoLinks for all the broken tests.

Original issue's description:
> XMLHttpRequest::overrideMimeType should throw InvalidStateError
> 
> Before this CL, XMLHttpRequest accepted |overrideMimeType| regardless
> of its state. This is not conformant to the current spec:
> http://www.w3.org/TR/XMLHttpRequest2/#the-overridemimetype-method
> 
> This CL changes it to throw InvalidStateError when the state
> is LOADING or DONE, which aligns to the spec.
> 
> BUG=402375
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=180440

TBR=tyoshino@chromium.org,abarth@chromium.org,haraken@chromium.org,kouhei@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=402375
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=180455

[Mike West, 2014-08-18 13:12:24]

Created Revert of XMLHttpRequest::overrideMimeType should throw
InvalidStateError

[commit-bot: I haz the power, 2014-08-18 13:12:45]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/mkwst@chromium.org/486703002/1

[commit-bot: I haz the power, 2014-08-18 13:13:32]

Committed patchset #1 (1) as 180455

[haraken, 2014-08-18 13:17:15]

LGTM, thanks for the revert.

[kouhei (in TOK), 2014-08-18 23:06:52]

lgtm