Renamed and restructured the protobuf definitions for the rtc_event_log graphs.

webrtc/tools/event_log_visualizer/graph.proto

-syntax = "proto2";
-option optimize_for = LITE_RUNTIME;
-package webrtc.protobuf_plot;
-
-enum PlotStyle {
-  UNDEFINED = 0;
-  LINE_GRAPH = 1;
-  LINE_DOT_GRAPH = 2;
-  BAR_GRAPH = 3;
-};
-
-message DataSet {
-  repeated float xvalues = 1;
-  repeated float yvalues = 2;
-  optional string label = 3;
-  optional PlotStyle style = 4;
-}
-
-message Plot {
-  repeated DataSet data_sets = 1;
-  optional float xaxis_min = 2;
-  optional float xaxis_max = 3;
-  optional string xaxis_label = 4;
-  optional float yaxis_min = 5;
-  optional float yaxis_max = 6;
-  optional string yaxis_label = 7;
-  optional string title = 8;
-}
-
-message PlotCollection {
-  repeated Plot plots = 1;
-}