Limit the DLL search path when loading FFmpeg binaries on Windows.
Limit the DLL search path when loading FFmpeg binaries on Windows.

This prevents us from accidentally loading other copies of FFmpeg during renderer process boot.

BUG=35857
TEST=install a real version of chrome (don't simply unzip a build).  close chrome.  create an empty file named C:\Windows\avutil-50.dll.  start chrome and go to www.youtube.com/html5.  no error dialog box should appear.  repeat these steps with an earlier version of chrome to verify that an error dialog box appears complaining about C:\Windows\avutil-50.dll.
Review URL: http://codereview.chromium.org/2856013

git-svn-id: http://src.chromium.org/svn/trunk/src@50617 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: ead0b5097bfd62f0575af107e2a03f325a69c563
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser