Have a look at this graph:
https://chromeperf.appspot.com/report?sid=7652282bac32b117c055caae9cee578a55fd431a2e7fe8dbaa6294b93576b832&rev=622298

Example commit from one of the teeth:
e17f9fb7f2bcfcb032621a25f9467eec1b990512 - Import wpt@2d42384cf21efd71843295d319c1bab85b3acf4a

Web Platform Tests do not affect the apk, so the reason for the sawtooth is probably to do with the fact that different machines are building the apk, and some of them behave differently from the others.

The different as shown by:
tools/binary_size/diagnose_bloat.py e17f9fb7f2bcfcb032621a25f9467eec1b990512 --cloud

Section Sizes (Total=634 bytes (634 bytes)):
    .dex: 128 bytes (128 bytes) (20.2%)
    .other: 506 bytes (506 bytes) (79.8%)
Dex:
            -8 entries fields
            -9 entries strings

513 symbols added (+), 417 changed (~), 521 removed (-), 923710 unchanged (not shown)
Added/Removed by section: .dex: -8 .dex.method: +0
Of changed symbols, 911 grew, 540 shrank
Number of unique symbols 572806 -> 572798 (-8)

Section Legend: t=.text, r=.rodata, R=.data.rel.ro, d=.data, b=.bss, x=.dex, m=.dex.method, p=.pak.translations, P=.pak.nontranslated, o=.other
Index | Running Total | Section@Address | Δ PSS (Δ size_without_padding) | Path
------------------------------------------------------------
+ 0)        908 (143.2%) m@0x0        908 (0->908)       chrome/android/java/src/org/chromium/chrome/browser/download/home/list/DateOrderedListMediator.java
               DateOrderedListMediator#lambda$shareItemsInternal$1$DateOrderedListMediator
- 1)          0 (0.0%)  m@0x0        -908 (908->0)      chrome/android/java/src/org/chromium/chrome/browser/download/home/list/DateOrderedListMediator.java
               DateOrderedListMediator#lambda$shareItemsInternal$419$DateOrderedListMediator
+ 2)        731 (115.3%) m@0x0        731 (0->731)       third_party/feed/src/src/main/java/com/google/android/libraries/feed/feedrequestmanager/FeedRequestManager.java
               FeedRequestManager#lambda$executeRequest$2$FeedRequestManager
- 3)          0 (0.0%)  m@0x0        -731 (731->0)      third_party/feed/src/src/main/java/com/google/android/libraries/feed/feedrequestmanager/FeedRequestManager.java
               FeedRequestManager#lambda$executeRequest$37$FeedRequestManager
+ 4)        558 (88.0%) m@0x0        558 (0->558)       chrome/android/java/src/org/chromium/chrome/browser/download/DownloadLocationDialogBridge.java
               DownloadLocationDialogBridge#lambda$showDialog$0$DownloadLocationDialogBridge
- 5)          0 (0.0%)  m@0x0        -558 (558->0)      chrome/android/java/src/org/chromium/chrome/browser/download/DownloadLocationDialogBridge.java
               DownloadLocationDialogBridge#lambda$showDialog$409$DownloadLocationDialogBridge
+ 6)        533 (84.1%) m@0x0        533 (0->533)       chrome/android/java/src/org/chromium/chrome/browser/offlinepages/downloads/OfflinePageDownloadBridge.java
               OfflinePageDownloadBridge#lambda$openItem$0$OfflinePageDownloadBridge
- 7)          0 (0.0%)  m@0x0        -533 (533->0)      chrome/android/java/src/org/chromium/chrome/browser/offlinepages/downloads/OfflinePageDownloadBridge.java
               OfflinePageDownloadBridge#lambda$openItem$540$OfflinePageDownloadBridge
+ 8)        529 (83.4%) m@0x0        529 (0->529)       third_party/feed/src/src/main/java/com/google/android/libraries/feed/feedmodelprovider/internal/ModelChildBinder.java
               ModelChildBinder#lambda$bindChildren$0$ModelChildBinder
- 9)          0 (0.0%)  m@0x0        -529 (529->0)      third_party/feed/src/src/main/java/com/google/android/libraries/feed/feedmodelprovider/internal/ModelChildBinder.java
               ModelChildBinder#lambda$bindChildren$34$ModelChildBinder
+ 10)       516 (81.4%) m@0x0        516 (0->516)       chrome/android/java/src/org/chromium/chrome/browser/ChromeActivity.java
               ChromeActivity#lambda$addOrEditBookmark$12$ChromeActivity
- 11)         0 (0.0%)  m@0x0        -516 (516->0)      chrome/android/java/src/org/chromium/chrome/browser/ChromeActivity.java
               ChromeActivity#lambda$addOrEditBookmark$19$ChromeActivity
~ 12)       506 (79.8%) o@0x0        506 (0->0)         {no path}
               Overhead: APK file
+ 13)       943 (148.7%) m@0x0        437 (0->437)       third_party/feed/src/src/main/java/com/google/android/libraries/feed/feedstore/internal/PersistentFeedStore.java
               PersistentFeedStore#lambda$triggerContentGc$1$PersistentFeedStore
- 14)       506 (79.8%) m@0x0        -437 (437->0)      third_party/feed/src/src/main/java/com/google/android/libraries/feed/feedstore/internal/PersistentFeedStore.java
               PersistentFeedStore#lambda$triggerContentGc$61$PersistentFeedStore
+ 15)       925 (145.9%) m@0x0        419 (0->419)       chrome/android/java/src/org/chromium/chrome/browser/preferences/download/DownloadDirectoryAdapter.java
               DownloadDirectoryAdapter#lambda$update$0$DownloadDirectoryAdapter
- 16)       506 (79.8%) m@0x0        -419 (419->0)      chrome/android/java/src/org/chromium/chrome/browser/preferences/download/DownloadDirectoryAdapter.java
               DownloadDirectoryAdapter#lambda$update$408$DownloadDirectoryAdapter
+ 17)       917 (144.6%) m@0x0        411 (0->411)       chrome/android/java/src/org/chromium/chrome/browser/widget/ListMenuButton.java
               ListMenuButton#lambda$onFinishInflate$0$ListMenuButton
- 18)       506 (79.8%) m@0x0        -411 (411->0)      chrome/android/java/src/org/chromium/chrome/browser/widget/ListMenuButton.java
               ListMenuButton#lambda$onFinishInflate$103$ListMenuButton
+ 19)       916 (144.5%) m@0x0        410 (0->410)       chrome/android/java/src/org/chromium/chrome/browser/incognito/IncognitoNotificationService.java
               IncognitoNotificationService#lambda$onHandleIntent$1$IncognitoNotificationService
- 20)       506 (79.8%) m@0x0        -410 (410->0)      chrome/android/java/src/org/chromium/chrome/browser/incognito/IncognitoNotificationService.java
               IncognitoNotificationService#lambda$onHandleIntent$504$IncognitoNotificationService
+ 21)       907 (143.1%) m@0x0        401 (0->401)       android_webview/java/src/org/chromium/android_webview/AwBrowserProcess.java
               AwBrowserProcess#lambda$start$0$AwBrowserProcess
- 22)       506 (79.8%) m@0x0        -401 (401->0)      android_webview/java/src/org/chromium/android_webview/AwBrowserProcess.java
               AwBrowserProcess#lambda$start$45$AwBrowserProcess
+ 23)       883 (139.3%) m@0x0        377 (0->377)       chrome/android/java/src/org/chromium/chrome/browser/download/OMADownloadHandler.java
               OMADownloadHandler#lambda$showOMAInfoDialog$0$OMADownloadHandler
- 24)       506 (79.8%) m@0x0        -377 (377->0)      chrome/android/java/src/org/chromium/chrome/browser/download/OMADownloadHandler.java
               OMADownloadHandler#lambda$showOMAInfoDialog$397$OMADownloadHandler
- 25)       155 (24.4%) m@0x0        -351 (351->0)      chrome/android/java/src/org/chromium/chrome/browser/download/DownloadManagerService.java



Looking at the subsequent commit, the diff is exactly reversed.
Probably something we'll need r8-team to investigate.