Added GL_CHROMIUM_trace_marker feature as well as gpu_toplevel markers.

gpu/GLES2/extensions/CHROMIUM/CHROMIUM_trace_marker.txt

+Name
+
+    CHROMIUM_trace_marker
+
+Name Strings
+
+    GL_CHROMIUM_trace_marker
+
+Version
+
+    Last Modifed Date: December 17, 2014
+
+Dependencies
+
+    OpenGL ES 2.0 is required.
+
+Overview
+
+    This extension lets you mark chromium style GPU traces. Each trace can
+    specify both a category name and a trace name which will be associated
+    with the trace.
+
+    Each trace's beginning is marked by TraceBeginCHROMIUM and the end can
+    optionally be marked by TraceEndCHROMIUM. If the trace's end is not marked,
+    the trace will automatically end with the commands of the traced graphics

[vmiura, 2014/12/18 21:49:39]

nit: "the trace will automatically end when the context is destroyed".

I'm not sure we should go into detail about pausing & resuming when the context
is Flushed here.

[David Yen, 2014/12/18 21:54:27]

Done.

+    context is finished.
+
+    Multiple traces can happen simultaneously, however traces act as a stack
+    and must be fully contained within one another. Therefore, you cannot
+    have overlapping traces.
+
+    Once a trace has been recorded, it is up to the application to decide
+    how the traces should be displayed.
+
+New Tokens
+
+    None
+
+New Procedures and Functions
+
+    void TraceBeginCHROMIUM(const char* category_name, const char* trace_name)
+
+    Marks the beginning of when a GPU trace should begin. Once the trace begins
+    it lasts until the current graphics context is finished or when
+    TraceEndCHROMIUM is called.
+
+
+    void TraceEndCHROMIUM()
+
+    Marks the last trace to end, this will signal the application to stop
+    tracing the previous trace.
+
+Errors
+
+    None.
+
+New State
+
+    None.
+
+Revision History
+
+    12/17/2014    Documented the extension
+