[content shell] add support for getting tests from the command line.

base/command_line.cc

Index: base/command_line.cc
diff --git a/base/command_line.cc b/base/command_line.cc
index 983181fc7a613edd133905a4759369ac61fee3a0..29467e3196b41a328f6de6e3eefaae26af7d2aed 100644
--- a/base/command_line.cc
+++ b/base/command_line.cc
@@ -50,7 +50,8 @@ bool IsSwitch(const CommandLine::StringType& string,
               CommandLine::StringType* switch_value) {
   switch_string->clear();
   switch_value->clear();
-  if (GetSwitchPrefixLength(string) == 0)
+  size_t prefix_length = GetSwitchPrefixLength(string);
+  if (prefix_length == 0 || prefix_length == string.length())
     return false;
 
   const size_t equals_position = string.find(kSwitchValueSeparator);