Add a content_shell flag to turn on partial site isolation.

content/shell/common/shell_switches.cc

Index: content/shell/common/shell_switches.cc
diff --git a/content/shell/common/shell_switches.cc b/content/shell/common/shell_switches.cc
index ab5ad9ab89ec258781e4e3846081e7108d1d982e..25c1070e4eb7b197f1f999ee0bb7de8fa6665bad 100644
--- a/content/shell/common/shell_switches.cc
+++ b/content/shell/common/shell_switches.cc
@@ -52,6 +52,14 @@ const char kEncodeBinary[] = "encode-binary";
 // and debugging of layout tests that rely on it.
 const char kExposeInternalsForTesting[] = "expose-internals-for-testing";
 
+// Enable site isolation (--site-per-process style isolation) for a subset of
+// sites. The argument is a wildcard pattern which will be matched against the
+// site URL to determine which sites to isolate. This can be used to isolate
+// just one top-level domain, or just one scheme. Example usages:
+//     --isolate-sites-for-testing=*.com
+//     --isolate-sites-for-testing=https://*
+const char kIsolateSitesForTesting[] = "isolate-sites-for-testing";
+
 // Registers additional font files on Windows (for fonts outside the usual
 // %WINDIR%\Fonts location). Multiple files can be used by separating them
 // with a semicolon (;).