Change instant event phase name from i to I

This matches current definition in the design doc[1] and unifies phase name between Chromium and Blink. Telemetry already expects both "i" and "I" values [2].

It used to be "I" in Chromium but it was changed in https://chromiumcodereview.appspot.com/12252058/

[1] https://docs.google.com/a/chromium.org/document/d/1CvAClvFfyA5R-PhYUmn5OOQtYMH4h6I0nSsKchNAySU/edit#
[2] https://code.google.com/p/chromium/codesearch#chromium/src/tools/telemetry/telemetry/timeline/trace_event_importer.py&q=%22elif%20phase%20==%20'I'%20or%20phase%20==%20'i':%22&sq=package:chromium&type=cs&l=252

BUG=None

Review URL: https://codereview.chromium.org/399423005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284644 0039d316-1c4b-4281-b951-d872f2087c98

base/debug/trace_event.h

@@ -942,7 +942,7 @@ TRACE_EVENT_API_CLASS_EXPORT extern \
 #define TRACE_EVENT_PHASE_BEGIN    ('B')
 #define TRACE_EVENT_PHASE_END      ('E')
 #define TRACE_EVENT_PHASE_COMPLETE ('X')
-#define TRACE_EVENT_PHASE_INSTANT  ('i')
+#define TRACE_EVENT_PHASE_INSTANT  ('I')
 #define TRACE_EVENT_PHASE_ASYNC_BEGIN ('S')
 #define TRACE_EVENT_PHASE_ASYNC_STEP_INTO  ('T')
 #define TRACE_EVENT_PHASE_ASYNC_STEP_PAST  ('p')