Fix crash when running chrome --install-from-webstore

We were using an invisible dummy WebContents to decide where to parent the
confirmation dialog. This crashes under Aura.

BUG=334524
TEST=manual, "chrome --install-from-webstore=gplegfbjlmmehdoakndmohflojccocli" should not crash

Review URL: https://codereview.chromium.org/160093002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250657 0039d316-1c4b-4281-b951-d872f2087c98
Create a separate context so that skia & gl_helper don't step on each others toes.

BUG=333603

Review URL: https://codereview.chromium.org/154263009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250658 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250658