Send authorization headers when calling Swarming backend.

appengine/swarming/doc/Magic-Values.md

 # Magic Values
 
 Describes magic values on the swarming server
 
 ## Introduction
 
 There are a few "magic" values in the isolate and swarming server. Also some
 dimensions and state values have special meaning.
 
 
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 ## Swarming
 
 ### Task execution environment
 
 When a Swarming bot is running a task, the following environment variables are
 always set:
 
   - `SWARMING_HEADLESS=1` is always set.
   - `SWARMING_BOT_ID` is set to the bot id.
+  - `SWARMING_TASK_ID` is set to the task id.
+  - `SWARMING_AUTH_PARAMS` is path to a file with task authentication tokens.
 
 The following environment variables may be set to alter bot behavior:
 
   - `SWARMING_EXTERNAL_BOT_SETUP=1` disables bot_config.setup_bot hook.
 
 
 ### dimensions
 
   - `id`: must be a single value in the list, which also must be unique. It's
     what uniquely identify the bot.
   - `quarantined`: if present, it specifies the bot self-quarantined, as it
     found out it needs manual sysadmin assistance before being able to accept
     any task. An example is that it doesn't enough free disk space.
 
 
 ### state
 
   - `cost_usd_hour`: reports the base cost of this bot in $USD/hour.
   - `lease_expiration_ts`: when set to an integer or floating point value,
     informs the server of the time (in UTC seconds since epoch) that the bot
     will disconnect from the server. The server will not allow the bot to
     reap any tasks projected to end after the bot disconnects.
   - `periodic_reboot_secs`: when set to a integer, instructs the server to send
     a reboot command after this period. The actual period is fuzzed with a 10%
     delta.
   - `quarantined`: has the same meaning than in `dimensions`. It's also
     supported as a state.