Add a fake cast extension for testing.

chrome/browser/extensions/api/tab_capture/tab_capture_api.cc

Index: chrome/browser/extensions/api/tab_capture/tab_capture_api.cc
diff --git a/chrome/browser/extensions/api/tab_capture/tab_capture_api.cc b/chrome/browser/extensions/api/tab_capture/tab_capture_api.cc
index 440c14fcc7168645c060e9b01107b36d63a8f39f..43ae06a3d2f69541a62469dc4e7c441eef761198 100644
--- a/chrome/browser/extensions/api/tab_capture/tab_capture_api.cc
+++ b/chrome/browser/extensions/api/tab_capture/tab_capture_api.cc
@@ -65,6 +65,8 @@ const char* const kChromecastExtensionIds[] = {
     "hfaagokkkhdbgiakmmlclaapfelnkoah",  // Canary
     "dliochdbjfkdbacpmhlcpmleaejidimm",  // Google Cast Beta
     "boadgeojelhgndaghljhdicfkmllpafd",  // Google Cast Stable
+    // Mock extension for testing, under /chrome/test/data/extensions/tray_cast
+    "hlgmmjhlnlapooncikdpiiokdjcdpjme",

[achuithb, 2015/07/08 22:23:55]

Personally I think a comment like
// Test cast extension
is sufficient, but up to you

[jdufault, 2015/07/09 19:30:02]

Done, using the string to search instead of hardcoding a path sounds much better
to me.

 };
 
 bool TabCaptureCaptureFunction::RunSync() {