Revert of Change WokerThread to use a blink::WebThread. (https://codereview.chromium.org/400153002/)

Reason for revert:
Failed compile on Linux Tests and Linux Tests (dbg) builders.
http://build.chromium.org/p/chromium.webkit/builders/Linux%20Tests/builds/38284
http://build.chromium.org/p/chromium.webkit/builders/Linux%20Tests%20%28dbg%29/builds/3339

../../mojo/services/html_viewer/blink_platform_impl.cc:183:14:error: allocating an object of abstract class type 'mojo::WebThreadImpl'
  return new WebThreadImpl(name);
             ^
../../third_party/WebKit/public/platform/WebThread.h:73:18: note: unimplemented pure virtual method 'setSharedTimerFiredFunction' in 'WebThreadImpl'
    virtual void setSharedTimerFiredFunction(SharedTimerFunction) = 0;
                 ^

Original issue's description:
> Change WokerThread to use a blink::WebThread.
> 
> BUG=301515
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=179117

TBR=jochen@chromium.org,ager@chromium.org,aandrey@chromium.org,yurys@chromium.org,oilpan-reviews@chromium.org,haraken@chromium.org,darin@chromium.org,nasko@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=301515

Review URL: https://codereview.chromium.org/430493002

git-svn-id: svn://svn.chromium.org/blink/trunk@179123 bbb929c8-8fbe-4397-9dbb-9b2b20218538