[Benchmarking] Add all tag for blink_perf.dom

From Pinpoint, end user can launch a job by selecting a benchmark together
with a story or a story tag. Though, there's no way to run all stories of
a benchmark. The reason behind this is that Pinpoint does not have enough
devices so it wants to limit the duration of each job. However, some
benchmarks are small and it won't be a big load even running all of their
stories.

Per Blink team's request, this CL adds "all" tag to two of its benchmarks:
blink_perf.dom

Bug: chromium:1023451
Change-Id: I334e3d3157b3c29c94e3de342d4d5e060948f712
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2028120Reviewed-by: Mikhail Khokhlov <khokhlov@google.com>
Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Commit-Queue: Caleb Rouleau <crouleau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736812}

tools/perf/benchmark.csv

@@ -9,7 +9,7 @@ blink_perf.bindings,"jbroman@chromium.org, yukishiino@chromium.org, haraken@chro
 blink_perf.canvas,"aaronhk@chromium.org, fserb@chromium.org",Blink>Canvas,https://bit.ly/blink-perf-benchmarks,
 blink_perf.css,"futhark@chromium.org, andruud@chromium.org",Blink>CSS,https://bit.ly/blink-perf-benchmarks,
 blink_perf.display_locking,vmpstr@chromium.org,Blink>Paint,https://bit.ly/blink-perf-benchmarks,
-blink_perf.dom,masonfreed@chromium.org,Blink>DOM,https://bit.ly/blink-perf-benchmarks,
+blink_perf.dom,masonfreed@chromium.org,Blink>DOM,https://bit.ly/blink-perf-benchmarks,all
 blink_perf.events,masonfreed@chromium.org,Blink>DOM,https://bit.ly/blink-perf-benchmarks,
 blink_perf.image_decoder,cblume@chromium.org,Internals>Images>Codecs,https://bit.ly/blink-perf-benchmarks,
 blink_perf.layout,"ikilpatrick@chromium.org, kojii@chromium.org",Blink>Layout,https://bit.ly/blink-perf-benchmarks,all

tools/perf/benchmarks/blink_perf.py

@@ -473,6 +473,7 @@ class BlinkPerfCanvas(_BlinkPerfBenchmark):
                 documentation_url='https://bit.ly/blink-perf-benchmarks')
 class BlinkPerfDOM(_BlinkPerfBenchmark):
   SUBDIR = 'dom'
+  TAGS = _BlinkPerfBenchmark.TAGS + ['all']
 
   @classmethod
   def Name(cls):