[Chromecast] Read LOGCAT_SIZE from BuildConfig

Modify LogcatExtractor to read LOGCAT_SIZE from BuildConfig,
so that different platforms can use different sizes.

Bug: internal b/69330105
Test: Send feedback
Change-Id: I6c78b8060eb1215c2665b990ba98fcbe3a35d067
Reviewed-on: https://chromium-review.googlesource.com/780149
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Commit-Queue: Steve Fung <stevefung@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518096}

diff --git a/chromecast/chromecast.gni b/chromecast/chromecast.gni
index de07990..28cb596 100644
--- a/chromecast/chromecast.gni
+++ b/chromecast/chromecast.gni

@@ -54,6 +54,9 @@
   # Set to true to keep Android activities alive when they are moved to the
   # background.
   enable_background_activities = false
+
+  # Set the number of lines for LogcatExtractor to send.
+  logcat_extractor_size = 512
 }
 
 declare_args() {