Update linux-perf bot dimensions

Update bot dimensions for linux-perf after OS upgrade from
Ubuntu 14.04 to 18.04.

Bug: 1162000
Change-Id: Ia2f40977120ccf0ac4b2037114c3d20ba5bdeb7a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2617240Reviewed-by: Wenbin Zhang <wenbinzhang@google.com>
Auto-Submit: John Chen <johnchen@chromium.org>
Commit-Queue: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841340}

testing/buildbot/chromium.perf.json

@@ -990,8 +990,8 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "gpu": "10de:1cb3-384.90",
-              "os": "Ubuntu-14.04",
+              "gpu": "10de:1cb3-440.100",
+              "os": "Ubuntu-18.04",
               "pool": "chrome.tests.perf",
               "synthetic_product_name": "PowerEdge R230 (Dell Inc.)"
             }

tools/perf/core/perf_data_generator.py

@@ -697,8 +697,8 @@ BUILDERS = {
         'platform':
         'linux',
         'dimension': {
-            'gpu': '10de:1cb3-384.90',
-            'os': 'Ubuntu-14.04',
+            'gpu': '10de:1cb3-440.100',
+            'os': 'Ubuntu-18.04',
             'pool': 'chrome.tests.perf',
             'synthetic_product_name': 'PowerEdge R230 (Dell Inc.)'
         },