[tracing] Add gpu.angle tracing category

This CL adds the "gpu.angle" category to the list of categories in
about:tracing. The "gpu.angle" category was enabled by default and could
not be disabled before.

R=eseckler@chromium.org

Change-Id: I243ffcdf1b170371b312e6895b1544cf1e20e551
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2349348Reviewed-by: Eric Seckler <eseckler@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796928}

base/trace_event/builtin_categories.h

@@ -82,6 +82,7 @@
   X("fonts")                                                             \
   X("GAMEPAD")                                                           \
   X("gpu")                                                               \
+  X("gpu.angle")                                                         \
   X("gpu.capture")                                                       \
   X("gpu.memory")                                                        \
   X("headless")                                                          \