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Chrome and WebView have supported basic Android ATrace tracing, but the
current approach has some shortcomings:

1) Android R changes the way atrace session activations are broadcast,
   breaking the code in WebView listening for activations
   (OnTraceEnabledChangeListener). This means you can't trace a running
   WebView app with atrace.

2) Neither Chrome nor WebView record early startup events to atrace,
   which means all events before the native library has loaded are
   lost.

3) It's not possible to specify trace categories via atrace, which means
   we need more cumbersome alternative solutions (i.e., command line
   flags) for startup tracing.

4) Writing ATrace events is only supported in WebView.

This patch reworks the ATrace integration to resolve these problems and
to align the Chrome and WebView implementations. In short, ATrace
session management is moved to the common (TraceEvent) layer, and Chrome
and WebView only differ by which trace tags they listen to (APP vs.
WEBVIEW).

We also add a way to declare trace categories through per-app tags:

$ atrace -a org.chromium.chrome,org.chromium.chrome/<category_filter>

(Note that the plain package name without any category filters must
always appear in the list on its own.)

Multiple categories can be separated with a double colon:

$ atrace -a org.chromium.chrome,org.chromium.chrome/cat1:cat2

Or by specifying the the app several times to get around the 91
character limit for each entry:

$ atrace -a org.chromium.chrome,\
            org.chromium.chrome/cat1,\
            org.chromium.chrome/cat2

Finally, to capture Java startup events into a tracing session
controlled by the system's Perfetto service instead of atrace, a
special "-atrace" category can be used to only write events into
Chrome's own tracing service instead of atrace. This way when we
connect to Perfetto later in the startup sequence and establish
the real tracing session, startup-related events can be flushed
into that session without emitting duplicate events into ATrace.

# Basic trace while Chrome is running. Generates ATrace with Chrome
# events.
TEST=atrace -a org.chromium.chrome

# Ditto for WebView.
TEST=atrace webview

# Startup trace -- open Chrome after running this. Results in an ATrace
# with Chrome pre- and post-startup events.
TEST=atrace -a org.chromium.chrome

# Ditto for WebView.
TEST=atrace webview

# Trace with custom categories. Results in a ATrace with only
# "cc" events from Chrome.
TEST=atrace -a org.chromium.chrome,org.chromium.chrome/-*:cc

# Ditto for WebView, using GMail as a test app.
TEST=atrace -a com.google.android.gm,com.google.android.gm/-*:cc webview

# Chrome-only trace. No Chrome events written in the resulting ATrace.
TEST=atrace -a org.chromium.chrome,org.chromium.chrome/-atrace

Bug: 1095587, b/160768681
Change-Id: Ia0bee252ce398804c00be8a4179241b75479bf5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2332664Reviewed-by: Bo <boliu@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Commit-Queue: Bo <boliu@chromium.org>
Auto-Submit: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798791}