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This is the first in a series of patches to implement tabs.removeCSS

A style sheet injected with tabs.insertCSS will have an internal
autogenerated "key" that will be passed to StyleEngine. In the case of
files, this will likely be just the fully qualified name of the file,
whereas in the case of code it could be the code string itself or a hash
digest of the code string. In either case, a call to tabs.removeCSS with
the same options will deterministically generate the same internal key,
thus enabling the removal of a previously injected style sheet without
the explicit use of IDs and such.

This patch lays the groundwork in StyleEngine that will enable us to
implement behavior similar to that observed in Firefox 57.0.

Changes:

 1.  WebStyleSheetId is now WebStyleSheetKey (WebString)
 2.  WebDocument::InsertStyleSheet now accepts an optional key as a
     parameter
 3.  StyleEngine::InjectSheet and StyleEngine::RemoveInjectedSheet now
     take StyleSheetKey (AtomicString) instead of WebStyleSheetId
 4.  Both WebDocument::RemoveInsertedStyleSheet and
     StyleEngine::RemoveInjectedSheet now take the CSS origin as a
     parameter
 5.  StyleEngine::RemoveInjectedSheet now removes only the last style
     sheet injected with the given key and only from the specified
     CSS origin

BUG=608854

Change-Id: Ie62bfcb0d8309b18cbf0d2cd119c74908e396767
Reviewed-on: https://chromium-review.googlesource.com/877663Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Manish Jethani <m.jethani@eyeo.com>
Cr-Commit-Position: refs/heads/master@{#537281}