Extract UI dependencies from ExtensionHost, part 3

1. Move WebContentsModalDialog support to ExtensionViewHost, as it is only
used by extension popups.
2. Break reliance on ChromeWebModalDialogManagerDelegate by providing our
own one-line IsWebContentsVisible() implementation.
3. Remove a bunch of OS_ANDROID ifdefs, since Android can use the components
module where web_modal stuff is implemented.

BUG=321341
TEST=browser_tests, esp. DownloadExtensionTest.DownloadExtensionTest_AcceptDanger

Review URL: https://codereview.chromium.org/92743002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238436 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238436
[rAC, OSX] Default to prod wallet.

R=estade@chromium.org
BUG=none

Review URL: https://codereview.chromium.org/100103002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238437 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238437