Fix off-by-one in ALSA control name determination.

Previously, given a device_id "plug:foo" alsa_util.cc would try to open
the control named ":foo". It will now open "foo".

This is safe because at this point we know we have already found a ':'
at pos1.

BUG=internal 35197382
TEST=manual
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2883313004
Cr-Commit-Position: refs/heads/master@{#472218}

media/audio/alsa/alsa_util.cc

@@ -54,9 +54,9 @@ static std::string DeviceNameToControlName(const std::string& device_name) {
     // deviceName: "front:CARD=Intel,DEV=0", controlName: "hw:CARD=Intel".
     // deviceName: "default:CARD=Intel", controlName: "CARD=Intel".
     size_t pos2 = device_name.find(',');
-    control_name = (pos2 == std::string::npos) ?
-        device_name.substr(pos1) :
-        kMixerPrefix + device_name.substr(pos1, pos2 - pos1);
+    control_name = (pos2 == std::string::npos)
+                       ? device_name.substr(pos1 + 1)
+                       : kMixerPrefix + device_name.substr(pos1, pos2 - pos1);
   }
 
   return control_name;